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Unpaid losses and loss expenses Unpaid Losses and loss expenses, claims development (Cumulative Net incurred Loss and Allocated Loss Adjustment Expense) (Details)
$ in Millions
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Dec. 31, 2012
USD ($)
Dec. 31, 2011
USD ($)
Dec. 31, 2010
USD ($)
Dec. 31, 2009
USD ($)
Dec. 31, 2008
USD ($)
Dec. 31, 2007
USD ($)
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net $ 45,304                  
North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 10,172                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 4,814                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 2,226                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 7,584                  
North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 33,251                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 17,665                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 1,726                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 17,312                  
North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 16,493                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 14,386                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 143                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 2,250                  
North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 6,241                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 4,711                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 241                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 1,771                  
North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 19,819                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 17,837                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 39                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 2,021                  
Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 11,747                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 6,405                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 339                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 5,681                  
Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 14,557                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 12,520                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 65                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 2,102                  
Global Reinsurance Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 1,432                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 1,278                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 0                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 154                  
Global Reinsurance Non-Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 4,049                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 2,814                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 357                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 1,592                  
Short-duration Insurance Contracts, Accident Year 2007 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 913 $ 933 $ 934 $ 944 $ 943 $ 976 $ 1,018 $ 1,026 $ 1,042 $ 1,068
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 171                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 641 625 606 580 550 502 451 386 281 121
Short-duration Insurance Contracts, Number of Reported Claims 351,000                  
Short-duration Insurance Contracts, Accident Year 2007 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,105 3,253 3,300 3,318 3,447 3,692 3,755 3,719 3,760 3,682
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 249                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,757 2,700 2,618 2,497 2,301 1,980 1,488 922 447 76
Short-duration Insurance Contracts, Number of Reported Claims 21,000                  
Short-duration Insurance Contracts, Accident Year 2007 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,139 1,138 1,139 1,138 1,137 1,151 1,168 1,181 1,190 1,271
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 0                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,135 1,134 1,133 1,130 1,126 1,124 1,117 1,076 987 563
Short-duration Insurance Contracts, Number of Reported Claims 906,000                  
Short-duration Insurance Contracts, Accident Year 2007 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 596 597 591 599 611 611 630 659 682 726
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 24                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 564 558 547 540 527 489 452 379 274 112
Short-duration Insurance Contracts, Number of Reported Claims 24,000                  
Short-duration Insurance Contracts, Accident Year 2007 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,398 1,398 1,400 1,402 1,411 1,416 1,431 1,460 1,538 1,549
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 6                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,387 1,387 1,384 1,377 1,370 1,353 1,328 1,267 1,182 843
Short-duration Insurance Contracts, Number of Reported Claims 127,000                  
Short-duration Insurance Contracts, Accident Year 2007 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 848 867 888 932 1,021 1,080 1,163 1,180 1,174 1,136
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 21                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 787 776 743 715 680 631 550 425 287 95
Short-duration Insurance Contracts, Number of Reported Claims 37,000                  
Short-duration Insurance Contracts, Accident Year 2007 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,184 1,177 1,178 1,181 1,165 1,189 1,191 1,209 1,214 1,224
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 5                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,164 1,159 1,152 1,147 1,140 1,124 1,091 1,032 861 426
Short-duration Insurance Contracts, Number of Reported Claims 183,000                  
Short-duration Insurance Contracts, Accident Year 2007 [Member] | Global Reinsurance Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 86 87 87 87 88 90 91 97 106 107
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 2                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 86 86 86 85 85 84 81 77 65 23
Short-duration Insurance Contracts, Number of Reported Claims 0                  
Short-duration Insurance Contracts, Accident Year 2007 [Member] | Global Reinsurance Non-Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 558 564 570 581 606 620 641 670 670 655
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 70                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 445 437 417 393 363 318 267 200 122 $ 52
Short-duration Insurance Contracts, Number of Reported Claims 1,470                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 932 948 952 952 966 991 997 993 1,018  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 185                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 627 602 574 542 500 436 370 276 125  
Short-duration Insurance Contracts, Number of Reported Claims 333,000                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,036 3,142 3,217 3,290 3,551 3,682 3,708 3,719 3,716  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 335                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,591 2,496 2,394 2,169 1,837 1,500 987 471 115  
Short-duration Insurance Contracts, Number of Reported Claims 21,000                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,845 1,846 1,859 1,849 1,845 1,836 1,852 1,880 1,927  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 29                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,813 1,813 1,807 1,800 1,797 1,766 1,708 1,607 929  
Short-duration Insurance Contracts, Number of Reported Claims 995,000                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 847 853 849 856 856 879 910 942 903  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 18                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 815 807 798 777 748 690 604 464 210  
Short-duration Insurance Contracts, Number of Reported Claims 25,000                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,659 1,659 1,667 1,675 1,693 1,722 1,746 1,777 1,777  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 6                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,648 1,644 1,642 1,636 1,620 1,584 1,518 1,406 974  
Short-duration Insurance Contracts, Number of Reported Claims 139,000                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,267 1,273 1,290 1,360 1,398 1,389 1,386 1,301 1,189  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 105                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,036 986 931 859 759 618 455 295 120  
Short-duration Insurance Contracts, Number of Reported Claims 37,000                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,269 1,280 1,284 1,287 1,302 1,294 1,318 1,360 1,364  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 14                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,233 1,233 1,225 1,215 1,206 1,183 1,120 985 479  
Short-duration Insurance Contracts, Number of Reported Claims 206,000                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Global Reinsurance Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 208 209 210 208 206 206 206 214 223  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 0                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 208 208 208 207 205 194 169 139 65  
Short-duration Insurance Contracts, Number of Reported Claims 0                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Global Reinsurance Non-Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 474 475 480 480 503 513 528 512 488  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 55                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 373 362 343 319 286 239 187 113 $ 46  
Short-duration Insurance Contracts, Number of Reported Claims 1,292                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 877 889 902 908 928 933 933 960    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 220                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 559 535 506 463 412 347 260 108    
Short-duration Insurance Contracts, Number of Reported Claims 283,000                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,060 3,130 3,205 3,454 3,556 3,599 3,630 3,682    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 447                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,461 2,307 2,118 1,813 1,460 979 425 86    
Short-duration Insurance Contracts, Number of Reported Claims 21,000                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,369 1,355 1,353 1,356 1,357 1,367 1,395 1,366    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 28                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,335 1,332 1,322 1,280 1,268 1,213 1,118 630    
Short-duration Insurance Contracts, Number of Reported Claims 1,067,000                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 546 548 557 593 640 665 710 718    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 16                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 520 511 500 471 432 378 281 106    
Short-duration Insurance Contracts, Number of Reported Claims 20,000                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,532 1,536 1,536 1,543 1,551 1,565 1,595 1,608    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 9                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,519 1,511 1,501 1,484 1,437 1,345 1,234 886    
Short-duration Insurance Contracts, Number of Reported Claims 125,000                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,191 1,195 1,300 1,411 1,414 1,400 1,357 1,214    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 124                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 950 860 791 734 642 505 330 119    
Short-duration Insurance Contracts, Number of Reported Claims 36,000                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,137 1,143 1,143 1,160 1,176 1,203 1,288 1,315    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 8                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,116 1,113 1,106 1,096 1,073 1,010 882 449    
Short-duration Insurance Contracts, Number of Reported Claims 234,000                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Global Reinsurance Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 100 100 100 103 105 105 113 99    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 0                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 98 98 98 96 95 90 79 41    
Short-duration Insurance Contracts, Number of Reported Claims 0                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Global Reinsurance Non-Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 354 365 383 401 409 404 404 356    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 35                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 278 265 246 224 191 151 109 $ 49    
Short-duration Insurance Contracts, Number of Reported Claims 921                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 972 994 1,004 1,008 1,003 988 985      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 254                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 612 587 548 495 414 303 125      
Short-duration Insurance Contracts, Number of Reported Claims 304,000                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,045 3,169 3,334 3,474 3,495 3,468 3,507      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 559                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,294 2,125 1,757 1,421 981 501 97      
Short-duration Insurance Contracts, Number of Reported Claims 20,000                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,420 1,424 1,432 1,442 1,486 1,568 1,509      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 15                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,401 1,400 1,394 1,374 1,335 1,231 703      
Short-duration Insurance Contracts, Number of Reported Claims 1,054,000                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 608 606 639 668 727 736 737      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 27                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 571 560 517 481 430 334 144      
Short-duration Insurance Contracts, Number of Reported Claims 21,000                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,823 1,828 1,832 1,836 1,853 1,876 1,868      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 11                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,803 1,792 1,770 1,727 1,669 1,521 1,152      
Short-duration Insurance Contracts, Number of Reported Claims 149,000                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,118 1,239 1,284 1,349 1,280 1,203 1,170      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 162                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 843 794 707 602 461 266 106      
Short-duration Insurance Contracts, Number of Reported Claims 38,000                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,392 1,400 1,413 1,424 1,424 1,449 1,434      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 30                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,334 1,328 1,316 1,278 1,218 1,028 523      
Short-duration Insurance Contracts, Number of Reported Claims 252,000                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Global Reinsurance Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 180 179 178 172 173 179 155      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 6                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 173 173 171 167 157 137 40      
Short-duration Insurance Contracts, Number of Reported Claims 0                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Global Reinsurance Non-Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 457 467 472 485 479 472 442      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 84                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 331 315 283 253 209 149 $ 71      
Short-duration Insurance Contracts, Number of Reported Claims 877                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 954 984 982 978 963 957        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 276                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 560 529 480 410 294 119        
Short-duration Insurance Contracts, Number of Reported Claims 287,000                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,360 3,472 3,539 3,525 3,498 3,437        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 856                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,310 2,045 1,638 1,069 538 111        
Short-duration Insurance Contracts, Number of Reported Claims 21,000                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,915 1,895 1,916 1,922 1,957 1,985        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 43                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,856 1,832 1,822 1,742 1,585 942        
Short-duration Insurance Contracts, Number of Reported Claims 1,045,000                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 651 659 675 703 717 696        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 40                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 585 561 522 455 335 131        
Short-duration Insurance Contracts, Number of Reported Claims 23,000                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,158 2,162 2,171 2,183 2,207 2,205        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 17                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,127 2,103 2,049 1,969 1,833 1,358        
Short-duration Insurance Contracts, Number of Reported Claims 168,000                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,062 1,140 1,217 1,222 1,232 1,239        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 240                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 686 610 511 382 240 88        
Short-duration Insurance Contracts, Number of Reported Claims 39,000                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,620 1,631 1,644 1,677 1,728 1,665        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,563 1,549 1,519 1,465 1,272 631        
Short-duration Insurance Contracts, Number of Reported Claims 268,000                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Global Reinsurance Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 222 221 220 224 224 227        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 217 214 198 175 151 75        
Short-duration Insurance Contracts, Number of Reported Claims 0                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Global Reinsurance Non-Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 458 467 472 474 461 449        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 67                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 330 304 270 227 171 $ 80        
Short-duration Insurance Contracts, Number of Reported Claims 796                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 932 967 957 940 956          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 321                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 483 435 365 272 111          
Short-duration Insurance Contracts, Number of Reported Claims 288,000                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,441 3,480 3,519 3,529 3,489          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1,168                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,953 1,535 1,028 532 114          
Short-duration Insurance Contracts, Number of Reported Claims 21,000                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,901 1,909 1,938 1,971 2,069          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 53                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,830 1,810 1,745 1,612 727          
Short-duration Insurance Contracts, Number of Reported Claims 1,027,000                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 633 668 682 714 748          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 60                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 538 484 413 307 108          
Short-duration Insurance Contracts, Number of Reported Claims 24,000                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,183 2,189 2,181 2,181 2,183          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 25                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,115 2,061 1,955 1,804 1,175          
Short-duration Insurance Contracts, Number of Reported Claims 173,000                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,269 1,278 1,257 1,196 1,253          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 373                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 677 569 422 243 74          
Short-duration Insurance Contracts, Number of Reported Claims 40,000                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,400 1,401 1,442 1,473 1,472          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 43                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,311 1,290 1,233 1,050 553          
Short-duration Insurance Contracts, Number of Reported Claims 283,000                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Global Reinsurance Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 141 142 149 154 175          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 4                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 129 125 117 95 29          
Short-duration Insurance Contracts, Number of Reported Claims 0                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Global Reinsurance Non-Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 425 440 440 437 440          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 30                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 334 301 260 202 $ 93          
Short-duration Insurance Contracts, Number of Reported Claims 677                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,047 1,043 1,031 1,013            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 374                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 499 415 286 107            
Short-duration Insurance Contracts, Number of Reported Claims 301,000                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,477 3,481 3,479 3,490            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1,588                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,489 1,089 468 101            
Short-duration Insurance Contracts, Number of Reported Claims 22,000                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,383 1,359 1,446 1,487            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 73                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,291 1,241 1,119 630            
Short-duration Insurance Contracts, Number of Reported Claims 1,068,000                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 621 632 640 619            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 121                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 452 372 292 115            
Short-duration Insurance Contracts, Number of Reported Claims 21,000                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,894 1,891 1,883 1,855            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 29                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,782 1,683 1,500 1,040            
Short-duration Insurance Contracts, Number of Reported Claims 126,000                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,274 1,231 1,219 1,229            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 537                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 555 413 260 87            
Short-duration Insurance Contracts, Number of Reported Claims 41,000                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,436 1,474 1,521 1,530            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 59                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,301 1,272 1,088 569            
Short-duration Insurance Contracts, Number of Reported Claims 281,000                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Global Reinsurance Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 105 109 117 117            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net (2)                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 103 98 85 38            
Short-duration Insurance Contracts, Number of Reported Claims 0                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Global Reinsurance Non-Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 361 362 363 359            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 54                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 247 206 158 $ 72            
Short-duration Insurance Contracts, Number of Reported Claims 417                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,129 1,111 1,106              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 526                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 408 296 113              
Short-duration Insurance Contracts, Number of Reported Claims 338,000                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,585 3,501 3,460              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1,969                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,124 576 116              
Short-duration Insurance Contracts, Number of Reported Claims 23,000                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,681 1,755 1,755              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 103                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,539 1,405 833              
Short-duration Insurance Contracts, Number of Reported Claims 1,095,000                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 651 658 655              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 176                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 387 289 113              
Short-duration Insurance Contracts, Number of Reported Claims 22,000                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,189 2,203 2,202              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 141                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,923 1,762 1,308              
Short-duration Insurance Contracts, Number of Reported Claims 134,000                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,285 1,286 1,242              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 626                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 459 289 114              
Short-duration Insurance Contracts, Number of Reported Claims 41,000                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,622 1,664 1,601              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 63                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,394 1,209 622              
Short-duration Insurance Contracts, Number of Reported Claims 269,000                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Global Reinsurance Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 142 142 125              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 6                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 127 112 56              
Short-duration Insurance Contracts, Number of Reported Claims 0                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Global Reinsurance Non-Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 373 367 367              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 69                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 240 199 $ 99              
Short-duration Insurance Contracts, Number of Reported Claims 428                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,157 1,179                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 588                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 302 117                
Short-duration Insurance Contracts, Number of Reported Claims 338,000                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,668 3,502                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 2,639                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 544 117                
Short-duration Insurance Contracts, Number of Reported Claims 26,000                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,828 1,839                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 281                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,334 712                
Short-duration Insurance Contracts, Number of Reported Claims 1,164,000                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 525 538                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 211                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 205 80                
Short-duration Insurance Contracts, Number of Reported Claims 20,000                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,546 2,491                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 213                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,081 1,497                
Short-duration Insurance Contracts, Number of Reported Claims 137,000                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,252 1,186                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 719                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 283 93                
Short-duration Insurance Contracts, Number of Reported Claims 41,000                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,812 1,705                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 192                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,315 692                
Short-duration Insurance Contracts, Number of Reported Claims 256,000                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Global Reinsurance Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 105 98                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 9                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 87 50                
Short-duration Insurance Contracts, Number of Reported Claims 0                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Global Reinsurance Non-Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 333 328                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 87                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 173 $ 95                
Short-duration Insurance Contracts, Number of Reported Claims 370                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,259                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 905                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 123                  
Short-duration Insurance Contracts, Number of Reported Claims 275,000                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,474                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 3,082                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 142                  
Short-duration Insurance Contracts, Number of Reported Claims 19,000                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,012                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 828                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 852                  
Short-duration Insurance Contracts, Number of Reported Claims 1,098,000                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 563                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 408                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 74                  
Short-duration Insurance Contracts, Number of Reported Claims 17,000                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,437                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 609                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,452                  
Short-duration Insurance Contracts, Number of Reported Claims 120,000                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,181                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 914                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 129                  
Short-duration Insurance Contracts, Number of Reported Claims 29,000                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,685                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 424                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 789                  
Short-duration Insurance Contracts, Number of Reported Claims 191,000                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Global Reinsurance Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 143                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 29                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 50                  
Short-duration Insurance Contracts, Number of Reported Claims 0                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Global Reinsurance Non-Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 256                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 141                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 63                  
Short-duration Insurance Contracts, Number of Reported Claims 118