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Insurance Claim Reserves (details) - Claim Development
$ in Millions
Dec. 31, 2016
USD ($)
item
Dec. 31, 2015
USD ($)
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                    
Total net liability (Audited) $ 40,951                  
Property casualty [Member]                    
Claims Development                    
Total net liability (Audited) 40,951                  
Business and International Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 9,254                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 5,253                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 4,001                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 3,033                  
Total net liability (Audited) 7,034                  
Business and International Insurance [Member] | Commercial property [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 4,108                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 3,406                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 702                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 164                  
Total net liability (Audited) 866                  
Business and International Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 17,153                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 14,044                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 3,109                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 305                  
Total net liability (Audited) 3,414                  
Business and International Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 6,255                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 4,169                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 2,086                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 184                  
Total net liability (Audited) 2,270                  
Business and International Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 21,999                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 11,788                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 10,211                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 5,228                  
Total net liability (Audited) 15,439                  
Business and International Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 6,509                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 5,122                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 1,387                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 95                  
Total net liability (Audited) 1,482                  
Bond & Specialty Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 5,754                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 3,786                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 1,968                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 74                  
Total net liability (Audited) 2,042                  
Bond & Specialty Insurance [Member] | Fidelity and surety [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 868                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 482                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 386                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 64                  
Total net liability (Audited) 450                  
Personal Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 11,476                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 9,452                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 2,024                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 253                  
Total net liability (Audited) 2,277                  
Personal Insurance [Member] | Homeowners (excluding Other) [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 7,857                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 7,136                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Disclosed Accident Years (Audited) 721                  
Liability for Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance, Before Disclosed Accident Years (Audited) 21                  
Total net liability (Audited) 742                  
Short-duration Insurance Contracts, Accident Year 2007 [Member] | Business and International Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 824 $ 865 $ 883 $ 888 $ 919 $ 968 $ 1,089 $ 1,087 $ 1,162 $ 1,136
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 726 713 697 682 632 549 467 316 134 32
IBNR Reserves Dec. 31, 2016 (Audited) $ 78                  
Cumulative Number of Reported Claims (Audited) | item 23,840                  
Short-duration Insurance Contracts, Accident Year 2007 [Member] | Business and International Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 1,369 1,376 1,369 1,373 1,375 1,398 1,402 1,364 1,430 1,490
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,312 1,307 1,296 1,278 1,256 1,208 1,110 982 824 498
IBNR Reserves Dec. 31, 2016 (Audited) $ 51                  
Cumulative Number of Reported Claims (Audited) | item 96,767                  
Short-duration Insurance Contracts, Accident Year 2007 [Member] | Business and International Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 1,341 1,328 1,340 1,358 1,373 1,390 1,448 1,484 1,519 1,554
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 936 910 880 845 802 747 683 589 450 216
IBNR Reserves Dec. 31, 2016 (Audited) $ 217                  
Cumulative Number of Reported Claims (Audited) | item 103,064                  
Short-duration Insurance Contracts, Accident Year 2007 [Member] | Business and International Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 597 601 608 611 609 607 608 616 626 632
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 578 570 561 547 524 488 450 410 355 228
IBNR Reserves Dec. 31, 2016 (Audited) $ 4                  
Cumulative Number of Reported Claims (Audited) | item 66,327                  
Short-duration Insurance Contracts, Accident Year 2007 [Member] | Bond & Specialty Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 454 462 467 479 511 551 582 638 571 584
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 415 410 400 393 373 357 303 229 134 $ 35
IBNR Reserves Dec. 31, 2016 (Audited) $ 4                  
Cumulative Number of Reported Claims (Audited) | item 11,018                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Business and International Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 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 2016) 799 759 734 694 615 497 359 154 35  
IBNR Reserves Dec. 31, 2016 (Audited) $ 88                  
Cumulative Number of Reported Claims (Audited) | item 25,385                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Business and International Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 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 2016) 1,617 1,602 1,581 1,551 1,490 1,396 1,264 1,103 712  
IBNR Reserves Dec. 31, 2016 (Audited) $ 47                  
Cumulative Number of Reported Claims (Audited) | item 108,382                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Business and International Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 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 2016) 1,162 1,130 1,088 1,036 961 875 752 571 274  
IBNR Reserves Dec. 31, 2016 (Audited) $ 237                  
Cumulative Number of Reported Claims (Audited) | item 107,565                  
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Business and International Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 642 646 651 655 658 665 670 685 681  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 622 616 603 579 543 501 460 406 256  
IBNR Reserves Dec. 31, 2016 (Audited) $ 5                  
Cumulative Number of Reported Claims (Audited) | item 71,803                  
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 2016) $ 631 643 672 712 716 697 743 769 579  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 590 579 562 529 471 387 281 157 $ 47  
IBNR Reserves Dec. 31, 2016 (Audited) $ 24                  
Cumulative Number of Reported Claims (Audited) | item 6,463                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Business and International Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 796 809 837 869 960 1,028 1,071 1,060    
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 667 643 613 543 446 314 167 35    
IBNR Reserves Dec. 31, 2016 (Audited) $ 95                  
Cumulative Number of Reported Claims (Audited) | item 25,457                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Business and International Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 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 2016) 1,449 1,436 1,408 1,360 1,264 1,121 958 603    
IBNR Reserves Dec. 31, 2016 (Audited) $ 42                  
Cumulative Number of Reported Claims (Audited) | item 103,198                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Business and International Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 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 2016) 1,235 1,193 1,137 1,065 961 828 623 288    
IBNR Reserves Dec. 31, 2016 (Audited) $ 272                  
Cumulative Number of Reported Claims (Audited) | item 104,229                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Business and International Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 690 696 702 714 714 707 718 724    
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 648 630 608 573 525 479 419 274    
IBNR Reserves Dec. 31, 2016 (Audited) $ 13                  
Cumulative Number of Reported Claims (Audited) | item 71,671                  
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 2016) $ 641 655 660 680 686 665 624 592    
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 592 563 497 460 390 310 167 $ 36    
IBNR Reserves Dec. 31, 2016 (Audited) $ 24                  
Cumulative Number of Reported Claims (Audited) | item 6,287                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Business and International Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 918 912 927 959 1,021 1,031 1,028      
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 756 702 629 487 324 139 35      
IBNR Reserves Dec. 31, 2016 (Audited) $ 108                  
Cumulative Number of Reported Claims (Audited) | item 27,678                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Business and International Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 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 2016) 1,798 1,763 1,698 1,579 1,395 1,180 709      
IBNR Reserves Dec. 31, 2016 (Audited) $ 51                  
Cumulative Number of Reported Claims (Audited) | item 111,586                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Business and International Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 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 2016) 1,385 1,321 1,246 1,133 978 750 341      
IBNR Reserves Dec. 31, 2016 (Audited) $ 354                  
Cumulative Number of Reported Claims (Audited) | item 116,837                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Business and International Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 706 715 724 735 721 722 715      
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 641 615 573 522 465 403 258      
IBNR Reserves Dec. 31, 2016 (Audited) $ 20                  
Cumulative Number of Reported Claims (Audited) | item 71,437                  
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 2016) $ 653 668 661 679 679 612 571      
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 597 565 482 396 291 152 $ 33      
IBNR Reserves Dec. 31, 2016 (Audited) $ 18                  
Cumulative Number of Reported Claims (Audited) | item 5,655                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Business and International Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 935 972 998 1,065 1,074 1,004        
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 725 660 539 355 187 47        
IBNR Reserves Dec. 31, 2016 (Audited) $ 146                  
Cumulative Number of Reported Claims (Audited) | item 27,210                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Business and International Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 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 2016) 2,156 2,088 1,979 1,803 1,573 1,060        
IBNR Reserves Dec. 31, 2016 (Audited) $ 66                  
Cumulative Number of Reported Claims (Audited) | item 125,358                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Business and International Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 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 2016) 1,583 1,487 1,365 1,185 911 420        
IBNR Reserves Dec. 31, 2016 (Audited) $ 430                  
Cumulative Number of Reported Claims (Audited) | item 135,061                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Business and International Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 669 678 682 686 696 723        
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 571 538 481 432 380 253        
IBNR Reserves Dec. 31, 2016 (Audited) $ 30                  
Cumulative Number of Reported Claims (Audited) | item 72,063                  
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 2016) $ 545 601 632 639 596 565        
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 447 414 324 249 143 $ 33        
IBNR Reserves Dec. 31, 2016 (Audited) $ 34                  
Cumulative Number of Reported Claims (Audited) | item 5,191                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business and International Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 892 913 935 985 989          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 589 489 295 150 32          
IBNR Reserves Dec. 31, 2016 (Audited) $ 184                  
Cumulative Number of Reported Claims (Audited) | item 24,384                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business and International Insurance [Member] | Commercial property [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 895 889 924 988 1,054          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 872 865 845 770 453          
IBNR Reserves Dec. 31, 2016 (Audited) $ 13                  
Cumulative Number of Reported Claims (Audited) | item 28,183                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business and International Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 1,888 1,888 1,903 1,883 1,885          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,699 1,590 1,424 1,246 795          
IBNR Reserves Dec. 31, 2016 (Audited) $ 88                  
Cumulative Number of Reported Claims (Audited) | item 104,419                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business and International Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 1,337 1,325 1,327 1,350 1,294          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,235 1,134 960 753 467          
IBNR Reserves Dec. 31, 2016 (Audited) $ 35                  
Cumulative Number of Reported Claims (Audited) | item 214,780                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business and International Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 2,445 2,456 2,457 2,456 2,447          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,536 1,394 1,217 940 443          
IBNR Reserves Dec. 31, 2016 (Audited) $ 519                  
Cumulative Number of Reported Claims (Audited) | item 133,417                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business and International Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 616 611 631 642 664          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 482 435 389 336 225          
IBNR Reserves Dec. 31, 2016 (Audited) $ 50                  
Cumulative Number of Reported Claims (Audited) | item 67,320                  
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 2016) $ 601 605 614 591 538          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 383 342 255 160 38          
IBNR Reserves Dec. 31, 2016 (Audited) $ 137                  
Cumulative Number of Reported Claims (Audited) | item 4,824                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 140 175 249 262 255          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 111 110 124 108 42          
IBNR Reserves Dec. 31, 2016 (Audited) $ 9                  
Cumulative Number of Reported Claims (Audited) | item 1,148                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Personal Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 2,428 2,432 2,448 2,454 2,417          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 2,376 2,311 2,189 1,983 1,503          
IBNR Reserves Dec. 31, 2016 (Audited) $ 15                  
Cumulative Number of Reported Claims (Audited) | item 793,669                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Personal Insurance [Member] | Homeowners (excluding Other) [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 2,019 2,018 2,029 2,056 2,136          
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 2,008 1,993 1,964 1,901 $ 1,508          
IBNR Reserves Dec. 31, 2016 (Audited) $ 1                  
Cumulative Number of Reported Claims (Audited) | item 259,006                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business and International Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 940 958 975 965            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 498 363 175 35            
IBNR Reserves Dec. 31, 2016 (Audited) $ 256                  
Cumulative Number of Reported Claims (Audited) | item 21,836                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business and International Insurance [Member] | Commercial property [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 731 737 755 789            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 703 683 610 389            
IBNR Reserves Dec. 31, 2016 (Audited) $ 10                  
Cumulative Number of Reported Claims (Audited) | item 22,141                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business and International Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 1,609 1,620 1,623 1,615            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,304 1,167 987 644            
IBNR Reserves Dec. 31, 2016 (Audited) $ 122                  
Cumulative Number of Reported Claims (Audited) | item 82,936                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business and International Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 1,245 1,240 1,236 1,235            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,039 884 675 435            
IBNR Reserves Dec. 31, 2016 (Audited) $ 67                  
Cumulative Number of Reported Claims (Audited) | item 197,041                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business and International Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 2,506 2,540 2,545 2,553            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,413 1,237 954 458            
IBNR Reserves Dec. 31, 2016 (Audited) $ 651                  
Cumulative Number of Reported Claims (Audited) | item 128,111                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business and International Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 685 701 717 723            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 494 439 388 260            
IBNR Reserves Dec. 31, 2016 (Audited) $ 56                  
Cumulative Number of Reported Claims (Audited) | item 71,183                  
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 2016) $ 630 606 565 510            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 352 252 154 34            
IBNR Reserves Dec. 31, 2016 (Audited) $ 204                  
Cumulative Number of Reported Claims (Audited) | item 4,371                  
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 2016) $ 146 199 246 240            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 131 128 113 37            
IBNR Reserves Dec. 31, 2016 (Audited) $ 6                  
Cumulative Number of Reported Claims (Audited) | item 1,006                  
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 2016) $ 2,044 2,049 2,095 2,108            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,935 1,814 1,628 1,251            
IBNR Reserves Dec. 31, 2016 (Audited) $ 36                  
Cumulative Number of Reported Claims (Audited) | item 694,650                  
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 2016) $ 1,375 1,365 1,397 1,488            
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,344 1,317 1,269 $ 994            
IBNR Reserves Dec. 31, 2016 (Audited) $ 4                  
Cumulative Number of Reported Claims (Audited) | item 149,373                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business and International Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 983 989 976              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 321 163 37              
IBNR Reserves Dec. 31, 2016 (Audited) $ 438                  
Cumulative Number of Reported Claims (Audited) | item 20,926                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business and International Insurance [Member] | Commercial property [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 836 860 936              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 775 710 464              
IBNR Reserves Dec. 31, 2016 (Audited) $ 15                  
Cumulative Number of Reported Claims (Audited) | item 21,490                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business and International Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 1,625 1,627 1,663              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,154 956 628              
IBNR Reserves Dec. 31, 2016 (Audited) $ 193                  
Cumulative Number of Reported Claims (Audited) | item 76,833                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business and International Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 1,168 1,166 1,165              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 821 618 397              
IBNR Reserves Dec. 31, 2016 (Audited) $ 131                  
Cumulative Number of Reported Claims (Audited) | item 184,067                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business and International Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 2,547 2,553 2,554              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,224 944 455              
IBNR Reserves Dec. 31, 2016 (Audited) $ 839                  
Cumulative Number of Reported Claims (Audited) | item 123,110                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business and International Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 682 678 665              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 445 382 252              
IBNR Reserves Dec. 31, 2016 (Audited) $ 83                  
Cumulative Number of Reported Claims (Audited) | item 67,935                  
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 2016) $ 563 571 549              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 239 150 38              
IBNR Reserves Dec. 31, 2016 (Audited) $ 232                  
Cumulative Number of Reported Claims (Audited) | item 4,182                  
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 2016) $ 165 212 223              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 111 96 58              
IBNR Reserves Dec. 31, 2016 (Audited) $ 32                  
Cumulative Number of Reported Claims (Audited) | item 992                  
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 2016) $ 1,981 1,994 2,014              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,763 1,564 1,193              
IBNR Reserves Dec. 31, 2016 (Audited) $ 75                  
Cumulative Number of Reported Claims (Audited) | item 669,990                  
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 2016) $ 1,453 1,450 1,515              
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,402 1,338 $ 1,053              
IBNR Reserves Dec. 31, 2016 (Audited) $ 9                  
Cumulative Number of Reported Claims (Audited) | item 151,517                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business and International Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 956 998                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 137 36                
IBNR Reserves Dec. 31, 2016 (Audited) $ 592                  
Cumulative Number of Reported Claims (Audited) | item 18,771                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business and International Insurance [Member] | Commercial property [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 750 786                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 615 376                
IBNR Reserves Dec. 31, 2016 (Audited) $ 28                  
Cumulative Number of Reported Claims (Audited) | item 19,859                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business and International Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 1,625 1,568                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 970 595                
IBNR Reserves Dec. 31, 2016 (Audited) $ 354                  
Cumulative Number of Reported Claims (Audited) | item 68,278                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business and International Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 1,215 1,198                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 658 409                
IBNR Reserves Dec. 31, 2016 (Audited) $ 246                  
Cumulative Number of Reported Claims (Audited) | item 179,963                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business and International Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 2,585 2,644                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 893 430                
IBNR Reserves Dec. 31, 2016 (Audited) $ 1,142                  
Cumulative Number of Reported Claims (Audited) | item 120,681                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business and International Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 610 597                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 346 228                
IBNR Reserves Dec. 31, 2016 (Audited) $ 112                  
Cumulative Number of Reported Claims (Audited) | item 58,238                  
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 2016) $ 524 528                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 141 38                
IBNR Reserves Dec. 31, 2016 (Audited) $ 272                  
Cumulative Number of Reported Claims (Audited) | item 3,814                  
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 2016) $ 191 217                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 75 32                
IBNR Reserves Dec. 31, 2016 (Audited) $ 79                  
Cumulative Number of Reported Claims (Audited) | item 768                  
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 2016) $ 2,244 2,186                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,768 1,319                
IBNR Reserves Dec. 31, 2016 (Audited) $ 224                  
Cumulative Number of Reported Claims (Audited) | item 755,762                  
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 2016) $ 1,454 1,438                
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,333 $ 994                
IBNR Reserves Dec. 31, 2016 (Audited) $ 28                  
Cumulative Number of Reported Claims (Audited) | item 144,367                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business and International Insurance [Member] | General liability [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 1,075                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 35                  
IBNR Reserves Dec. 31, 2016 (Audited) $ 908                  
Cumulative Number of Reported Claims (Audited) | item 13,810                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business and International Insurance [Member] | Commercial property [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 896                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 441                  
IBNR Reserves Dec. 31, 2016 (Audited) $ 131                  
Cumulative Number of Reported Claims (Audited) | item 19,327                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business and International Insurance [Member] | Commercial multi-peril [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 1,662                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 585                  
IBNR Reserves Dec. 31, 2016 (Audited) $ 658                  
Cumulative Number of Reported Claims (Audited) | item 56,471                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business and International Insurance [Member] | Automobile [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 1,290                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 416                  
IBNR Reserves Dec. 31, 2016 (Audited) $ 516                  
Cumulative Number of Reported Claims (Audited) | item 173,790                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business and International Insurance [Member] | Workers' compensation [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 2,768                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 421                  
IBNR Reserves Dec. 31, 2016 (Audited) $ 1,651                  
Cumulative Number of Reported Claims (Audited) | item 108,357                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business and International Insurance [Member] | International - Canada [Member] | Property casualty [Member]                    
Claims Development                    
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) $ 612                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 295                  
IBNR Reserves Dec. 31, 2016 (Audited) $ 102                  
Cumulative Number of Reported Claims (Audited) | item 54,948                  
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 2016) $ 512                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 30                  
IBNR Reserves Dec. 31, 2016 (Audited) $ 405                  
Cumulative Number of Reported Claims (Audited) | item 2,676                  
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 2016) $ 226                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 54                  
IBNR Reserves Dec. 31, 2016 (Audited) $ 128                  
Cumulative Number of Reported Claims (Audited) | item 595                  
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 2016) $ 2,779                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,610                  
IBNR Reserves Dec. 31, 2016 (Audited) $ 646                  
Cumulative Number of Reported Claims (Audited) | item 839,962                  
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 2016) $ 1,556                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Audited for 2016) 1,049                  
IBNR Reserves Dec. 31, 2016 (Audited) $ 307                  
Cumulative Number of Reported Claims (Audited) | item 129,630