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Unpaid losses and loss expenses Unpaid Losses and loss expenses, claims development (Cumulative Net incurred Loss and Allocated Loss Adjustment Expense) (Details)
$ in Millions
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Dec. 31, 2012
USD ($)
Dec. 31, 2011
USD ($)
Dec. 31, 2010
USD ($)
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net $ 46,062                  
North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 11,846                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 5,100                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 2,668                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 9,414                  
North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 34,245                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 19,320                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 1,522                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 16,447                  
North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 5,227                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 3,566                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 252                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 1,913                  
North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 18,124                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 16,376                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 11                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 1,759                  
North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 24,271                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 21,773                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 27                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 2,525                  
Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 12,380                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 6,835                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 432                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 5,977                  
Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 19,023                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 16,743                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 97                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 2,377                  
Global Reinsurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 3,052                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 2,178                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 303                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 1,177                  
Global Reinsurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 2,214                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 1,965                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 6                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 255                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 999 $ 1,018 $ 1,020 $ 1,028 $ 1,052 $ 1,064 $ 1,065 $ 1,050 $ 1,037 $ 1,049
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 223                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 684 666 641 617 592 551 493 411 300 123
Cumulative Number of Reported Claims 303,000                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,983 2,993 3,105 3,125 3,247 3,416 3,556 3,597 3,579 3,574
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 202                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,716 2,659 2,525 2,424 2,257 1,892 1,558 1,108 611 126
Cumulative Number of Reported Claims 18,000                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 481 484 493 480 478 506 546 601 607 613
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 16                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 452 453 449 443 433 392 363 322 236 97
Cumulative Number of Reported Claims 15,000                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,394 1,394 1,404 1,410 1,415 1,422 1,424 1,461 1,537 1,501
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,391 1,391 1,395 1,394 1,391 1,382 1,357 1,320 1,222 723
Cumulative Number of Reported Claims 1,057,000                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,820 1,821 1,820 1,823 1,828 1,832 1,835 1,853 1,876 1,868
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 6                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,812 1,810 1,809 1,803 1,791 1,770 1,727 1,668 1,521 1,151
Cumulative Number of Reported Claims 146,000                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,149 1,142 1,136 1,141 1,265 1,316 1,379 1,308 1,263 1,183
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 68                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 983 946 903 850 801 712 605 462 265 102
Cumulative Number of Reported Claims 37,000                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,582 1,584 1,582 1,599 1,612 1,626 1,632 1,643 1,669 1,647
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 6                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,562 1,550 1,545 1,544 1,537 1,524 1,486 1,424 1,226 671
Cumulative Number of Reported Claims 518,000                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Global Reinsurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 373 387 400 414 424 430 441 430 419 399
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 23                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 320 315 306 291 274 249 220 179 125 56
Cumulative Number of Reported Claims 802                  
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Global Reinsurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 217 218 219 218 218 216 212 218 228 194
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 0                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 214 210 210 207 209 199 193 182 156 $ 55
Cumulative Number of Reported Claims 102                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 988 1,009 1,012 1,022 1,053 1,049 1,046 1,030 1,037  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 233                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 640 616 595 567 533 484 411 294 119  
Cumulative Number of Reported Claims 286,000                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,190 3,312 3,380 3,494 3,590 3,660 3,626 3,582 3,496  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 299                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,824 2,738 2,657 2,474 2,212 1,803 1,208 651 160  
Cumulative Number of Reported Claims 18,000                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 512 510 516 524 533 548 581 589 580  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 24                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 486 480 466 461 437 400 341 235 86  
Cumulative Number of Reported Claims 15,000                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,833 1,832 1,832 1,837 1,833 1,853 1,875 1,932 1,958  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 10                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,822 1,819 1,813 1,808 1,785 1,775 1,715 1,571 938  
Cumulative Number of Reported Claims 1,051,000                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,156 2,156 2,157 2,158 2,162 2,170 2,182 2,207 2,205  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 8                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,146 2,143 2,136 2,126 2,103 2,049 1,969 1,833 1,358  
Cumulative Number of Reported Claims 166,000                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 988 991 1,042 1,054 1,117 1,200 1,210 1,218 1,211  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 35                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 848 815 764 691 612 513 384 240 87  
Cumulative Number of Reported Claims 37,000                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,810 1,814 1,824 1,832 1,843 1,861 1,900 1,956 1,871  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 3                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,773 1,773 1,769 1,761 1,746 1,716 1,660 1,460 758  
Cumulative Number of Reported Claims 544,000                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Global Reinsurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 401 407 413 417 427 432 428 414 407  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 27                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 331 324 311 291 267 236 195 146 70  
Cumulative Number of Reported Claims 659                  
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Global Reinsurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 259 259 259 260 258 258 268 270 269  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 256 254 253 251 246 228 204 174 $ 85  
Cumulative Number of Reported Claims 132                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 977 986 989 1,011 1,040 1,030 1,011 1,050    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 275                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 592 574 532 486 436 365 271 111    
Cumulative Number of Reported Claims 287,000                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,231 3,326 3,422 3,520 3,560 3,609 3,624 3,548    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 430                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,615 2,499 2,324 2,090 1,678 1,171 655 166    
Cumulative Number of Reported Claims 18,000                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 507 508 519 520 560 577 605 633    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 3                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 493 486 470 435 386 319 222 69    
Cumulative Number of Reported Claims 15,000                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,847 1,841 1,844 1,856 1,861 1,880 1,913 2,030    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 3                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,839 1,813 1,819 1,792 1,764 1,696 1,575 713    
Cumulative Number of Reported Claims 1,035,000                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,192 2,186 2,184 2,183 2,189 2,181 2,181 2,183    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 20                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,161 2,161 2,147 2,115 2,061 1,955 1,804 1,175    
Cumulative Number of Reported Claims 170,000                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,255 1,265 1,285 1,294 1,297 1,279 1,217 1,246    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 137                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 939 897 826 689 577 428 245 74    
Cumulative Number of Reported Claims 38,000                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,556 1,561 1,577 1,585 1,591 1,646 1,686 1,696    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 14                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,517 1,515 1,502 1,493 1,470 1,412 1,226 681    
Cumulative Number of Reported Claims 556,000                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Global Reinsurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 372 370 371 378 393 390 382 385    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 10                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 334 322 307 292 260 221 167 77    
Cumulative Number of Reported Claims 457                  
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Global Reinsurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 184 184 187 189 191 200 210 230    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 180 179 177 172 166 156 130 $ 45    
Cumulative Number of Reported Claims 113                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,037 1,073 1,086 1,127 1,122 1,108 1,109      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 309                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 616 587 553 506 422 286 107      
Cumulative Number of Reported Claims 299,000                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,118 3,212 3,426 3,528 3,538 3,538 3,543      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 500                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,371 2,230 2,005 1,595 1,191 547 130      
Cumulative Number of Reported Claims 17,000                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 461 462 468 515 522 530 526      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 29                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 418 411 385 348 270 197 69      
Cumulative Number of Reported Claims 17,000                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,334 1,337 1,337 1,356 1,333 1,420 1,430      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 3                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,329 1,321 1,308 1,282 1,234 1,135 649      
Cumulative Number of Reported Claims 1,072,000                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,938 1,931 1,918 1,894 1,890 1,882 1,854      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 26                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,890 1,879 1,837 1,781 1,682 1,499 1,040      
Cumulative Number of Reported Claims 122,000                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,136 1,193 1,226 1,272 1,229 1,233 1,237      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 139                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 865 798 699 558 414 261 85      
Cumulative Number of Reported Claims 38,000                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,609 1,621 1,651 1,656 1,703 1,770 1,778      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 27                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,562 1,553 1,534 1,497 1,466 1,273 698      
Cumulative Number of Reported Claims 574,000                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Global Reinsurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 316 323 330 329 328 326 320      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 20                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 268 259 241 222 186 143 65      
Cumulative Number of Reported Claims 341                  
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Global Reinsurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 140 140 143 142 147 159 161      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 0                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 135 135 132 129 120 102 $ 46      
Cumulative Number of Reported Claims 121                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,100 1,163 1,215 1,217 1,201 1,207        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 395                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 566 532 484 410 295 113        
Cumulative Number of Reported Claims 336,000                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,467 3,652 3,713 3,671 3,582 3,532        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 792                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,440 2,200 1,802 1,249 679 164        
Cumulative Number of Reported Claims 17,000                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 538 555 596 581 583 594        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 45                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 473 454 391 317 220 80        
Cumulative Number of Reported Claims 17,000                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,547 1,546 1,555 1,576 1,658 1,642        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 7                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,543 1,528 1,502 1,481 1,370 818        
Cumulative Number of Reported Claims 1,100,000                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,143 2,156 2,142 2,189 2,203 2,202        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 19                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,103 2,076 2,031 1,922 1,762 1,308        
Cumulative Number of Reported Claims 132,000                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,167 1,249 1,333 1,317 1,308 1,238        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 208                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 786 704 591 461 287 111        
Cumulative Number of Reported Claims 39,000                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,804 1,814 1,851 1,862 1,920 1,852        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 15                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,741 1,727 1,696 1,632 1,423 758        
Cumulative Number of Reported Claims 549,000                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Global Reinsurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 346 343 341 338 333 332        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 39                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 276 264 248 217 184 91        
Cumulative Number of Reported Claims 382                  
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Global Reinsurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 180 181 183 180 180 164        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 3                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 171 169 163 152 129 $ 65        
Cumulative Number of Reported Claims 101                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,217 1,279 1,276 1,259 1,282          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 500                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 564 501 418 301 116          
Cumulative Number of Reported Claims 334,000                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,939 3,971 3,814 3,705 3,556          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1,232                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,289 1,854 1,205 605 138          
Cumulative Number of Reported Claims 19,000                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 458 515 501 470 486          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 51                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 370 304 214 137 47          
Cumulative Number of Reported Claims 15,000                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,602 1,635 1,647 1,742 1,733          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 17                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,570 1,554 1,486 1,341 725          
Cumulative Number of Reported Claims 1,170,000                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,559 2,540 2,557 2,546 2,491          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 30                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,475 2,388 2,267 2,081 1,497          
Cumulative Number of Reported Claims 135,000                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,286 1,311 1,288 1,259 1,164          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 287                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 780 661 484 281 86          
Cumulative Number of Reported Claims 41,000                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,999 2,017 2,051 2,075 1,952          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 38                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,881 1,858 1,778 1,546 852          
Cumulative Number of Reported Claims 571,000                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Global Reinsurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 308 300 299 288 284          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 33                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 232 217 191 159 90          
Cumulative Number of Reported Claims 298                  
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Global Reinsurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 153 161 161 154 146          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 3                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 146 142 132 103 $ 56          
Cumulative Number of Reported Claims 115                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,378 1,383 1,361 1,366            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 673                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 529 452 326 122            
Cumulative Number of Reported Claims 304,000                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,801 3,688 3,591 3,530            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1,279                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,974 1,335 662 171            
Cumulative Number of Reported Claims 20,000                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 524 527 502 504            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 136                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 323 246 145 52            
Cumulative Number of Reported Claims 15,000                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,778 1,797 1,887 1,907            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 16                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,729 1,653 1,502 845            
Cumulative Number of Reported Claims 1,291,000                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,479 2,541 2,532 2,436            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 78                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,311 2,208 2,049 1,451            
Cumulative Number of Reported Claims 138,000                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,385 1,357 1,291 1,191            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 428                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 667 520 316 123            
Cumulative Number of Reported Claims 42,000                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,018 2,040 2,052 2,050            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 17                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,938 1,865 1,670 1,015            
Cumulative Number of Reported Claims 567,000                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Global Reinsurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 233 234 226 222            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 30                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 159 142 113 57            
Cumulative Number of Reported Claims 341                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Global Reinsurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 190 188 186 180            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 12                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 169 158 131 $ 56            
Cumulative Number of Reported Claims 182                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,399 1,380 1,412              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 783                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 437 313 120              
Cumulative Number of Reported Claims 339,000                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,577 3,495 3,319              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1,818                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,161 616 161              
Cumulative Number of Reported Claims 21,000                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 577 566 531              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 174                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 312 175 66              
Cumulative Number of Reported Claims 16,000                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,503 2,605 2,701              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 71                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,301 2,085 978              
Cumulative Number of Reported Claims 1,374,000                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,998 3,066 3,031              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 171                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,664 2,517 1,696              
Cumulative Number of Reported Claims 142,000                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,335 1,286 1,185              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 495                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 520 314 96              
Cumulative Number of Reported Claims 41,000                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,220 2,238 2,198              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 46                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,005 1,830 1,046              
Cumulative Number of Reported Claims 577,000                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Global Reinsurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 219 214 213              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 45                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 122 100 46              
Cumulative Number of Reported Claims 529                  
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Global Reinsurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 453 423 396              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 10                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 402 322 $ 191              
Cumulative Number of Reported Claims 309                  
Short-duration Insurance Contracts, Accident Year 2018 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,360 1,359                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 788                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 329 130                
Cumulative Number of Reported Claims 362,000                  
Short-duration Insurance Contracts, Accident Year 2018 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,490 3,371                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 2,170                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 754 189                
Cumulative Number of Reported Claims 24,000                  
Short-duration Insurance Contracts, Accident Year 2018 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 563 535                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 298                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 169 74                
Cumulative Number of Reported Claims 15,000                  
Short-duration Insurance Contracts, Accident Year 2018 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,237 2,050                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 182                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,823 1,026                
Cumulative Number of Reported Claims 1,551,000                  
Short-duration Insurance Contracts, Accident Year 2018 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,033 3,006                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 295                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,545 1,924                
Cumulative Number of Reported Claims 148,000                  
Short-duration Insurance Contracts, Accident Year 2018 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,333 1,283                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 789                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 325 109                
Cumulative Number of Reported Claims 40,000                  
Short-duration Insurance Contracts, Accident Year 2018 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,244 2,153                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 124                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,726 994                
Cumulative Number of Reported Claims 622,000                  
Short-duration Insurance Contracts, Accident Year 2018 [Member] | Global Reinsurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 246 244                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 65                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 96 41                
Cumulative Number of Reported Claims 589                  
Short-duration Insurance Contracts, Accident Year 2018 [Member] | Global Reinsurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 297 285                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net (6)                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 257 $ 94                
Cumulative Number of Reported Claims 212                  
Short-Duration Insurance Contract, Accident Year 2019 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,391                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 997                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 143                  
Cumulative Number of Reported Claims 246,000                  
Short-Duration Insurance Contract, Accident Year 2019 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,449                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 3,005                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 176                  
Cumulative Number of Reported Claims 25,000                  
Short-Duration Insurance Contract, Accident Year 2019 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 606                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 428                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 70                  
Cumulative Number of Reported Claims 14,000                  
Short-Duration Insurance Contract, Accident Year 2019 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,049                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 587                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,029                  
Cumulative Number of Reported Claims 1,446,000                  
Short-Duration Insurance Contract, Accident Year 2019 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,953                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 725                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,666                  
Cumulative Number of Reported Claims 116,000                  
Short-Duration Insurance Contract, Accident Year 2019 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,346                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1,011                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 122                  
Cumulative Number of Reported Claims 32,000                  
Short-Duration Insurance Contract, Accident Year 2019 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 2,181                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 376                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,038                  
Cumulative Number of Reported Claims 608,000                  
Short-Duration Insurance Contract, Accident Year 2019 [Member] | Global Reinsurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 238                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 130                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 40                  
Cumulative Number of Reported Claims 219                  
Short-Duration Insurance Contract, Accident Year 2019 [Member] | Global Reinsurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 141                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 73                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 35                  
Cumulative Number of Reported Claims 32