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Insurance Claim Reserves (details) - Claim Development
$ in Millions
Dec. 31, 2017
USD ($)
claims
Dec. 31, 2016
USD ($)
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 ($)
Dec. 31, 2008
USD ($)
Claims Development                    
Total net liability (Audited) $ 42,540                  
Property casualty [Member]                    
Claims Development                    
Total net liability (Audited) 42,540                  
Business Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 9,434                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 5,469                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 3,965                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 2,887                  
Total net liability (Audited) 6,852                  
Business Insurance [Member] | Commercial property [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 4,377                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 3,500                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 877                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 163                  
Total net liability (Audited) 1,040                  
Business Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 17,511                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 14,394                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 3,117                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 370                  
Total net liability (Audited) 3,487                  
Business Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 6,404                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 4,195                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 2,209                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 197                  
Total net liability (Audited) 2,406                  
Business Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 23,093                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 12,558                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 10,535                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 5,277                  
Total net liability (Audited) 15,812                  
Bond & Specialty Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 5,743                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 3,831                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 1,912                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 83                  
Total net liability (Audited) 1,995                  
Bond & Specialty Insurance [Member] | Fidelity and surety [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 939                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 540                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 399                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 61                  
Total net liability (Audited) 460                  
Personal Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 12,374                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 9,971                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 2,403                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 244                  
Total net liability (Audited) 2,647                  
Personal Insurance [Member] | Homeowners (excluding Other) [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 8,153                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 7,043                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 1,110                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 15                  
Total net liability (Audited) 1,125                  
Personal Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 4,293                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 3,577                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 716                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 44                  
Total net liability (Audited) 760                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Business Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 936 $ 935 $ 931 $ 946 $ 994 $ 1,041 $ 1,079 $ 1,222 $ 1,209 $ 1,143
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 817 799 759 734 694 615 497 359 154 35
IBNR Reserves Dec. 31, 2017 (Audited) $ 81                  
Cumulative Number of Reported Claims (Audited) | claims 25,462                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Business Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,680 1,681 1,688 1,674 1,684 1,674 1,688 1,683 1,674 1,725
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,625 1,617 1,602 1,581 1,551 1,490 1,396 1,264 1,103 712
IBNR Reserves Dec. 31, 2017 (Audited) $ 45                  
Cumulative Number of Reported Claims (Audited) | claims 108,522                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Business Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,607 1,617 1,617 1,621 1,634 1,639 1,683 1,734 1,745 1,714
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,189 1,162 1,130 1,088 1,036 961 875 752 571 274
IBNR Reserves Dec. 31, 2017 (Audited) $ 219                  
Cumulative Number of Reported Claims (Audited) | claims 107,851                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Bond & Specialty Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 637 631 643 672 712 716 697 743 769 579
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 593 590 579 562 529 471 387 281 157 47
IBNR Reserves Dec. 31, 2017 (Audited) $ 18                  
Cumulative Number of Reported Claims (Audited) | claims 6,468                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Personal Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 424 424 423 428 434 428 422 421 433 429
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 416 413 409 401 386 363 340 315 281 $ 184
Cumulative Number of Reported Claims (Audited) | claims 54,281                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Business Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 783 796 809 837 869 960 1,028 1,071 1,060  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 689 667 643 613 543 446 314 167 35  
IBNR Reserves Dec. 31, 2017 (Audited) $ 71                  
Cumulative Number of Reported Claims (Audited) | claims 25,551                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Business Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,500 1,509 1,514 1,514 1,511 1,498 1,501 1,506 1,484  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,457 1,449 1,436 1,408 1,360 1,264 1,121 958 603  
IBNR Reserves Dec. 31, 2017 (Audited) $ 30                  
Cumulative Number of Reported Claims (Audited) | claims 103,307                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Business Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,736 1,750 1,775 1,766 1,753 1,753 1,746 1,778 1,799  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,274 1,235 1,193 1,137 1,065 961 828 623 288  
IBNR Reserves Dec. 31, 2017 (Audited) $ 241                  
Cumulative Number of Reported Claims (Audited) | claims 104,348                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Bond & Specialty Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 631 641 655 660 680 686 665 624 592  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 595 592 563 497 460 390 310 167 36  
IBNR Reserves Dec. 31, 2017 (Audited) $ 16                  
Cumulative Number of Reported Claims (Audited) | claims 6,290                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Personal Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 481 489 488 496 505 496 490 481 494  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 471 468 456 438 412 380 351 308 $ 205  
IBNR Reserves Dec. 31, 2017 (Audited) $ 4                  
Cumulative Number of Reported Claims (Audited) | claims 55,162                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Business Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 908 918 912 927 959 1,021 1,031 1,028    
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 781 756 702 629 487 324 139 35    
IBNR Reserves Dec. 31, 2017 (Audited) $ 90                  
Cumulative Number of Reported Claims (Audited) | claims 27,810                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Business Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,885 1,898 1,892 1,895 1,861 1,832 1,826 1,711    
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,819 1,798 1,763 1,698 1,579 1,395 1,180 709    
IBNR Reserves Dec. 31, 2017 (Audited) $ 40                  
Cumulative Number of Reported Claims (Audited) | claims 111,756                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Business Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 2,021 2,055 2,052 2,049 2,056 2,035 2,042 1,886    
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,430 1,385 1,321 1,246 1,133 978 750 341    
IBNR Reserves Dec. 31, 2017 (Audited) $ 298                  
Cumulative Number of Reported Claims (Audited) | claims 116,911                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Bond & Specialty Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 653 653 668 661 679 679 612 571    
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 623 597 565 482 396 291 152 33    
IBNR Reserves Dec. 31, 2017 (Audited) $ 16                  
Cumulative Number of Reported Claims (Audited) | claims 5,667                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Personal Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 499 506 510 519 531 516 505 503    
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 475 463 445 413 382 341 302 $ 198    
IBNR Reserves Dec. 31, 2017 (Audited) $ 7                  
Cumulative Number of Reported Claims (Audited) | claims 54,924                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Business Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 913 935 972 998 1,065 1,074 1,004      
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 762 725 660 539 355 187 47      
IBNR Reserves Dec. 31, 2017 (Audited) $ 93                  
Cumulative Number of Reported Claims (Audited) | claims 27,327                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Business Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 2,283 2,287 2,296 2,286 2,269 2,244 2,235      
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 2,193 2,156 2,088 1,979 1,803 1,573 1,060      
IBNR Reserves Dec. 31, 2017 (Audited) $ 52                  
Cumulative Number of Reported Claims (Audited) | claims 125,522                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Business Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 2,363 2,385 2,379 2,350 2,347 2,303 2,284      
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,652 1,583 1,487 1,365 1,185 911 420      
IBNR Reserves Dec. 31, 2017 (Audited) $ 380                  
Cumulative Number of Reported Claims (Audited) | claims 136,325                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Bond & Specialty Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 520 545 601 632 639 596 565      
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 476 447 414 324 249 143 33      
IBNR Reserves Dec. 31, 2017 (Audited) $ 3                  
Cumulative Number of Reported Claims (Audited) | claims 5,207                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Personal Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 436 442 449 457 461 453 475      
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 403 384 361 325 290 258 $ 182      
IBNR Reserves Dec. 31, 2017 (Audited) $ 13                  
Cumulative Number of Reported Claims (Audited) | claims 55,788                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 905 892 913 935 985 989        
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 699 589 489 295 150 32        
IBNR Reserves Dec. 31, 2017 (Audited) $ 111                  
Cumulative Number of Reported Claims (Audited) | claims 24,632                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,867 1,888 1,888 1,903 1,883 1,885        
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,752 1,699 1,590 1,424 1,246 795        
IBNR Reserves Dec. 31, 2017 (Audited) $ 58                  
Cumulative Number of Reported Claims (Audited) | claims 104,646                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 2,453 2,445 2,456 2,457 2,456 2,447        
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,629 1,536 1,394 1,217 940 443        
IBNR Reserves Dec. 31, 2017 (Audited) $ 474                  
Cumulative Number of Reported Claims (Audited) | claims 136,451                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Bond & Specialty Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 599 601 605 614 591 538        
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 419 383 342 255 160 38        
IBNR Reserves Dec. 31, 2017 (Audited) $ 126                  
Cumulative Number of Reported Claims (Audited) | claims 4,838                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Personal Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 393 410 412 429 427 450        
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 345 327 298 271 239 $ 171        
IBNR Reserves Dec. 31, 2017 (Audited) $ 23                  
Cumulative Number of Reported Claims (Audited) | claims 51,215                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 927 940 958 975 965          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 639 498 363 175 35          
IBNR Reserves Dec. 31, 2017 (Audited) $ 166                  
Cumulative Number of Reported Claims (Audited) | claims 22,200                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business Insurance [Member] | Commercial property [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 729 731 737 755 789          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 713 703 683 610 389          
IBNR Reserves Dec. 31, 2017 (Audited) $ 6                  
Cumulative Number of Reported Claims (Audited) | claims 22,165                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,591 1,609 1,620 1,623 1,615          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,410 1,304 1,167 987 644          
IBNR Reserves Dec. 31, 2017 (Audited) $ 68                  
Cumulative Number of Reported Claims (Audited) | claims 83,371                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,254 1,245 1,240 1,236 1,235          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,154 1,039 884 675 435          
IBNR Reserves Dec. 31, 2017 (Audited) $ 27                  
Cumulative Number of Reported Claims (Audited) | claims 197,224                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 2,463 2,506 2,540 2,545 2,553          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,525 1,413 1,237 954 458          
IBNR Reserves Dec. 31, 2017 (Audited) $ 558                  
Cumulative Number of Reported Claims (Audited) | claims 128,957                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Bond & Specialty Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 654 630 606 565 510          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 400 352 252 154 34          
IBNR Reserves Dec. 31, 2017 (Audited) $ 200                  
Cumulative Number of Reported Claims (Audited) | claims 4,418                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 141 146 199 246 240          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 135 131 128 113 37          
Cumulative Number of Reported Claims (Audited) | claims 1,014                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Personal Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 2,039 2,044 2,049 2,095 2,108          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,992 1,935 1,814 1,628 1,251          
IBNR Reserves Dec. 31, 2017 (Audited) $ 16                  
Cumulative Number of Reported Claims (Audited) | claims 694,733                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Personal Insurance [Member] | Homeowners (excluding Other) [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,376 1,375 1,365 1,397 1,488          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,360 1,344 1,317 1,269 994          
IBNR Reserves Dec. 31, 2017 (Audited) $ 3                  
Cumulative Number of Reported Claims (Audited) | claims 149,430                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Personal Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 459 473 485 494 501          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 382 348 314 281 $ 200          
IBNR Reserves Dec. 31, 2017 (Audited) $ 24                  
Cumulative Number of Reported Claims (Audited) | claims 54,248                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 948 983 989 976            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 515 321 163 37            
IBNR Reserves Dec. 31, 2017 (Audited) $ 255                  
Cumulative Number of Reported Claims (Audited) | claims 21,703                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business Insurance [Member] | Commercial property [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 835 836 860 936            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 803 775 710 464            
IBNR Reserves Dec. 31, 2017 (Audited) $ 10                  
Cumulative Number of Reported Claims (Audited) | claims 21,548                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,617 1,625 1,627 1,663            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,328 1,154 956 628            
IBNR Reserves Dec. 31, 2017 (Audited) $ 99                  
Cumulative Number of Reported Claims (Audited) | claims 77,731                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,184 1,168 1,166 1,165            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 987 821 618 397            
IBNR Reserves Dec. 31, 2017 (Audited) $ 62                  
Cumulative Number of Reported Claims (Audited) | claims 184,476                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 2,476 2,547 2,553 2,554            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,399 1,224 944 455            
IBNR Reserves Dec. 31, 2017 (Audited) $ 681                  
Cumulative Number of Reported Claims (Audited) | claims 123,713                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Bond & Specialty Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 518 563 571 549            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 312 239 150 38            
IBNR Reserves Dec. 31, 2017 (Audited) $ 123                  
Cumulative Number of Reported Claims (Audited) | claims 4,290                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 136 165 212 223            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 127 111 96 58            
IBNR Reserves Dec. 31, 2017 (Audited) $ 3                  
Cumulative Number of Reported Claims (Audited) | claims 1,042                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Personal Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,985 1,981 1,994 2,014            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,879 1,763 1,564 1,193            
IBNR Reserves Dec. 31, 2017 (Audited) $ 33                  
Cumulative Number of Reported Claims (Audited) | claims 670,329                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Personal Insurance [Member] | Homeowners (excluding Other) [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,457 1,453 1,450 1,515            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,425 1,402 1,338 1,053            
IBNR Reserves Dec. 31, 2017 (Audited) $ 6                  
Cumulative Number of Reported Claims (Audited) | claims 151,664                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Personal Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 448 460 459 443            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 342 312 274 $ 196            
IBNR Reserves Dec. 31, 2017 (Audited) $ 30                  
Cumulative Number of Reported Claims (Audited) | claims 52,263                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 923 956 998              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 336 137 36              
IBNR Reserves Dec. 31, 2017 (Audited) $ 361                  
Cumulative Number of Reported Claims (Audited) | claims 20,236                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business Insurance [Member] | Commercial property [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 741 750 786              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 681 615 376              
IBNR Reserves Dec. 31, 2017 (Audited) $ 15                  
Cumulative Number of Reported Claims (Audited) | claims 20,069                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,593 1,625 1,568              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,144 970 595              
IBNR Reserves Dec. 31, 2017 (Audited) $ 191                  
Cumulative Number of Reported Claims (Audited) | claims 70,291                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,248 1,215 1,198              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 896 658 409              
IBNR Reserves Dec. 31, 2017 (Audited) $ 126                  
Cumulative Number of Reported Claims (Audited) | claims 181,269                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 2,505 2,585 2,644              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,154 893 430              
IBNR Reserves Dec. 31, 2017 (Audited) $ 906                  
Cumulative Number of Reported Claims (Audited) | claims 121,855                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Bond & Specialty Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 486 524 528              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 234 141 38              
IBNR Reserves Dec. 31, 2017 (Audited) $ 170                  
Cumulative Number of Reported Claims (Audited) | claims 4,060                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 179 191 217              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 87 75 32              
IBNR Reserves Dec. 31, 2017 (Audited) $ 60                  
Cumulative Number of Reported Claims (Audited) | claims 810                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Personal Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 2,236 2,244 2,186              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,985 1,768 1,319              
IBNR Reserves Dec. 31, 2017 (Audited) $ 101                  
Cumulative Number of Reported Claims (Audited) | claims 757,333                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Personal Insurance [Member] | Homeowners (excluding Other) [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,461 1,454 1,438              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,395 1,333 994              
IBNR Reserves Dec. 31, 2017 (Audited) $ 17                  
Cumulative Number of Reported Claims (Audited) | claims 144,930                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Personal Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 372 373 373              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 262 234 $ 167              
IBNR Reserves Dec. 31, 2017 (Audited) $ 43                  
Cumulative Number of Reported Claims (Audited) | claims 45,182                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,058 1,075                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 191 35                
IBNR Reserves Dec. 31, 2017 (Audited) $ 655                  
Cumulative Number of Reported Claims (Audited) | claims 17,680                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business Insurance [Member] | Commercial property [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 863 896                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 685 441                
IBNR Reserves Dec. 31, 2017 (Audited) $ 53                  
Cumulative Number of Reported Claims (Audited) | claims 22,055                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,623 1,662                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 950 585                
IBNR Reserves Dec. 31, 2017 (Audited) $ 358                  
Cumulative Number of Reported Claims (Audited) | claims 65,985                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,319 1,290                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 698 416                
IBNR Reserves Dec. 31, 2017 (Audited) $ 293                  
Cumulative Number of Reported Claims (Audited) | claims 190,036                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 2,690 2,768                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 873 421                
IBNR Reserves Dec. 31, 2017 (Audited) $ 1,128                  
Cumulative Number of Reported Claims (Audited) | claims 121,455                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Bond & Specialty Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 511 512                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 141 30                
IBNR Reserves Dec. 31, 2017 (Audited) $ 250                  
Cumulative Number of Reported Claims (Audited) | claims 3,976                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 239 226                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 121 54                
IBNR Reserves Dec. 31, 2017 (Audited) $ 45                  
Cumulative Number of Reported Claims (Audited) | claims 823                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Personal Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 2,791 2,779                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 2,203 1,610                
IBNR Reserves Dec. 31, 2017 (Audited) $ 279                  
Cumulative Number of Reported Claims (Audited) | claims 919,301                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Personal Insurance [Member] | Homeowners (excluding Other) [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,547 1,556                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,392 1,049                
IBNR Reserves Dec. 31, 2017 (Audited) $ 69                  
Cumulative Number of Reported Claims (Audited) | claims 143,163                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Personal Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 423 373                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 293 $ 218                
IBNR Reserves Dec. 31, 2017 (Audited) $ 55                  
Cumulative Number of Reported Claims (Audited) | claims 45,500                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Business Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,133                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 40                  
IBNR Reserves Dec. 31, 2017 (Audited) $ 968                  
Cumulative Number of Reported Claims (Audited) | claims 12,538                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Business Insurance [Member] | Commercial property [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,209                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 618                  
IBNR Reserves Dec. 31, 2017 (Audited) $ 166                  
Cumulative Number of Reported Claims (Audited) | claims 21,686                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Business Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,872                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 716                  
IBNR Reserves Dec. 31, 2017 (Audited) $ 617                  
Cumulative Number of Reported Claims (Audited) | claims 59,838                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Business Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 1,399                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 460                  
IBNR Reserves Dec. 31, 2017 (Audited) $ 588                  
Cumulative Number of Reported Claims (Audited) | claims 178,825                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Business Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 2,779                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 433                  
IBNR Reserves Dec. 31, 2017 (Audited) $ 1,747                  
Cumulative Number of Reported Claims (Audited) | claims 107,131                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Bond & Specialty Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 534                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 38                  
IBNR Reserves Dec. 31, 2017 (Audited) $ 415                  
Cumulative Number of Reported Claims (Audited) | claims 2,811                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 244                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 70                  
IBNR Reserves Dec. 31, 2017 (Audited) $ 132                  
Cumulative Number of Reported Claims (Audited) | claims 611                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Personal Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 3,323                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,912                  
IBNR Reserves Dec. 31, 2017 (Audited) $ 844                  
Cumulative Number of Reported Claims (Audited) | claims 970,143                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Personal Insurance [Member] | Homeowners (excluding Other) [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 2,312                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 1,471                  
IBNR Reserves Dec. 31, 2017 (Audited) $ 457                  
Cumulative Number of Reported Claims (Audited) | claims 152,146                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Personal Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) $ 358                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2017) 188                  
IBNR Reserves Dec. 31, 2017 (Audited) $ 44                  
Cumulative Number of Reported Claims (Audited) | claims 43,885