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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)
number in Thousands, $ in Millions
 12 Months Ended
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 ($)
Dec. 31, 2009
USD ($)
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net $ 45,847                  
PPD adjustments 63                  
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 11,613                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 5,004                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 2,574                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 9,183                  
North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 34,078                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 19,079                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 1,486                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 16,485                  
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 5,109                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 3,512                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 287                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 1,884                  
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 17,206                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 15,357                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 22                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 1,871                  
North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 22,911                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 20,610                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 18                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 2,319                  
North America Personal P&C Insurance [Member] | Catastrophe [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease) 200                  
Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 12,201                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 6,751                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 383                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 5,833                  
Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 18,107                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 15,890                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 48                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 2,265                  
Global Reinsurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 3,103                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 2,236                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 351                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 1,218                  
Global Reinsurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 2,176                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 1,799                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented 13                  
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net 390                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net 964 $ 965 $ 972 $ 966 $ 977 $ 980 $ 990 $ 997 $ 998 $ 1,029
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 218                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 633 617 597 550 519 475 416 348 258 107
Short-duration Insurance Contract, Cumulative Number of Reported Claims 282                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | North America Commercial P&C Insurance - Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 3,082 3,096 3,237 3,309 3,386 3,636 3,737 3,764 3,777 3,791
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 221                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,766 2,725 2,673 2,541 2,354 2,017 1,670 1,159 587 134
Short-duration Insurance Contract, Cumulative Number of Reported Claims 20                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | North America Commercial P&C Insurance - Other Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 452 445 450 452 458 492 535 555 589 599
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 $ 431 427 422 413 401 373 336 287 206 70
Short-duration Insurance Contract, Cumulative Number of Reported Claims 15                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | North America Commercial P&C Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,183 1,185 1,186 1,189 1,189 1,196 1,213 1,243 1,298 1,302
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 $ 1,178 1,178 1,178 1,176 1,168 1,160 1,146 1,122 1,032 618
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1,124                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | North America Personal P&C Insurance [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,527 1,529 1,530 1,534 1,534 1,541 1,549 1,563 1,593 1,606
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 5                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,520 1,519 1,517 1,509 1,499 1,482 1,435 1,343 1,233 884
Short-duration Insurance Contract, Cumulative Number of Reported Claims 125                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Overseas General Insurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,138 1,152 1,204 1,205 1,309 1,422 1,421 1,414 1,367 1,231
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 29                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,007 978 952 861 791 733 641 502 327 117
Short-duration Insurance Contract, Cumulative Number of Reported Claims 38                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,303 1,301 1,302 1,313 1,313 1,331 1,350 1,380 1,464 1,492
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 0                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,287 1,283 1,284 1,281 1,275 1,265 1,241 1,177 1,043 572
Short-duration Insurance Contract, Cumulative Number of Reported Claims 516                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Global Reinsurance - Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 303 313 318 328 344 363 367 360 348 316
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 11                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 262 255 239 226 208 187 154 116 79 34
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1                  
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Global Reinsurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 137 138 138 138 140 142 149 150 170 139
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 $ 133 133 133 133 132 130 128 121 105 $ 52
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 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 248                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 666 641 617 592 551 493 411 300 123  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 303                  
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 $ 2,991 3,103 3,123 3,245 3,413 3,554 3,594 3,576 3,571  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 234                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,658 2,523 2,423 2,257 1,891 1,557 1,107 611 126  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 19                  
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 $ 483 492 480 478 506 545 600 607 613  
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 $ 452 448 443 433 392 363 321 236 97  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 15                  
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 $ 1,393 1,403 1,409 1,413 1,421 1,423 1,459 1,535 1,500  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 0                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,390 1,393 1,393 1,389 1,381 1,356 1,319 1,221 722  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1,058                  
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 1,819 1,821 1,827 1,830 1,834 1,852 1,875 1,866  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 8                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,809 1,807 1,801 1,790 1,768 1,726 1,667 1,519 1,151  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 149                  
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 $ 1,139 1,134 1,138 1,260 1,312 1,375 1,304 1,259 1,180  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 83                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 942 899 847 797 709 603 461 264 102  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 40                  
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 $ 1,575 1,573 1,590 1,603 1,617 1,623 1,635 1,660 1,638  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 13                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,541 1,535 1,534 1,528 1,515 1,477 1,415 1,218 664  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 560                  
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 $ 386 399 413 423 429 440 429 418 398  
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 $ 314 306 291 273 249 220 179 124 56  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1                  
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 $ 222 223 222 221 219 215 221 232 197  
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 $ 214 214 211 213 203 197 186 160 $ 56  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 1,008 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 271                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 616 595 567 533 484 411 294 119    
Short-duration Insurance Contract, Cumulative Number of Reported Claims 286                  
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 $ 3,309 3,377 3,492 3,588 3,658 3,623 3,579 3,494    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 473                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,736 2,655 2,473 2,211 1,802 1,207 651 160    
Short-duration Insurance Contract, Cumulative Number of Reported Claims 20                  
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 $ 510 516 524 532 548 580 589 580    
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 $ 479 465 460 436 400 341 235 86    
Short-duration Insurance Contract, Cumulative Number of Reported Claims 16                  
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 $ 1,831 1,831 1,835 1,831 1,852 1,873 1,930 1,956    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 11                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,817 1,812 1,807 1,783 1,773 1,714 1,570 938    
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1,052                  
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 $ 2,154 2,155 2,156 2,160 2,169 2,181 2,205 2,203    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 9                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,141 2,134 2,125 2,101 2,048 1,968 1,831 1,357    
Short-duration Insurance Contract, Cumulative Number of Reported Claims 168                  
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 1,038 1,052 1,116 1,199 1,209 1,216 1,210    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 62                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 811 760 688 610 511 382 239 87    
Short-duration Insurance Contract, Cumulative Number of Reported Claims 41                  
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 $ 1,808 1,818 1,826 1,838 1,855 1,894 1,951 1,861    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 2                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,766 1,762 1,754 1,739 1,709 1,654 1,453 753    
Short-duration Insurance Contract, Cumulative Number of Reported Claims 578                  
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 $ 406 412 416 425 430 426 411 404    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 34                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 323 310 290 266 235 195 145 70    
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1                  
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 $ 261 261 262 261 260 270 272 271    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 2                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 257 255 253 248 230 205 174 $ 85    
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 986 989 1,011 1,040 1,030 1,011 1,050      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 292                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 574 532 486 436 365 271 111      
Short-duration Insurance Contract, Cumulative Number of Reported Claims 287                  
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 $ 3,323 3,419 3,517 3,557 3,606 3,622 3,546      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 626                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,497 2,322 2,089 1,677 1,170 654 166      
Short-duration Insurance Contract, Cumulative Number of Reported Claims 20                  
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 $ 508 518 519 560 576 605 633      
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 $ 486 470 435 386 319 222 69      
Short-duration Insurance Contract, Cumulative Number of Reported Claims 16                  
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 $ 1,840 1,842 1,854 1,860 1,878 1,911 2,029      
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 $ 1,812 1,817 1,790 1,762 1,694 1,573 713      
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1,036                  
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 $ 2,185 2,182 2,182 2,187 2,179 2,179 2,181      
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 $ 2,160 2,146 2,113 2,059 1,954 1,803 1,174      
Short-duration Insurance Contract, Cumulative Number of Reported Claims 173                  
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 $ 1,258 1,278 1,297 1,301 1,283 1,220 1,252      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 166                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 888 818 683 572 424 244 73      
Short-duration Insurance Contract, Cumulative Number of Reported Claims 43                  
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 $ 1,557 1,572 1,583 1,590 1,644 1,683 1,694      
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 $ 1,509 1,497 1,488 1,465 1,407 1,220 676      
Short-duration Insurance Contract, Cumulative Number of Reported Claims 599                  
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 $ 368 369 376 391 388 380 383      
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 $ 321 306 290 259 220 166 76      
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 184 187 189 190 200 210 230      
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 $ 179 176 171 165 155 129 $ 44      
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 1,073 1,086 1,127 1,122 1,108 1,109        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 358                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 587 553 506 422 286 107        
Short-duration Insurance Contract, Cumulative Number of Reported Claims 299                  
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 $ 3,209 3,423 3,526 3,536 3,536 3,541        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 748                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,229 2,004 1,594 1,190 547 129        
Short-duration Insurance Contract, Cumulative Number of Reported Claims 19                  
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 $ 461 468 515 522 530 526        
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 $ 411 385 348 270 196 68        
Short-duration Insurance Contract, Cumulative Number of Reported Claims 18                  
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 1,335 1,354 1,331 1,418 1,428        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 13                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,319 1,306 1,280 1,233 1,134 648        
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1,073                  
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 $ 1,927 1,915 1,890 1,887 1,879 1,851        
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 $ 1,876 1,834 1,778 1,679 1,496 1,038        
Short-duration Insurance Contract, Cumulative Number of Reported Claims 126                  
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 $ 1,192 1,227 1,283 1,240 1,243 1,247        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 221                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 796 695 559 414 260 85        
Short-duration Insurance Contract, Cumulative Number of Reported Claims 44                  
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 $ 1,619 1,649 1,659 1,705 1,773 1,780        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 42                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,550 1,531 1,495 1,464 1,271 695        
Short-duration Insurance Contract, Cumulative Number of Reported Claims 620                  
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 $ 321 328 327 327 324 318        
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 $ 258 240 221 185 142 64        
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 140 142 141 146 158 160        
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 $ 134 132 129 119 102 $ 46        
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 1,163 1,215 1,217 1,201 1,207          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 465                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 532 484 410 295 113          
Short-duration Insurance Contract, Cumulative Number of Reported Claims 336                  
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 $ 3,649 3,711 3,668 3,580 3,529          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1,148                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,200 1,802 1,249 679 164          
Short-duration Insurance Contract, Cumulative Number of Reported Claims 19                  
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 $ 554 596 580 582 594          
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 $ 454 391 317 220 80          
Short-duration Insurance Contract, Cumulative Number of Reported Claims 17                  
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 $ 1,544 1,554 1,574 1,656 1,640          
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 $ 1,527 1,501 1,479 1,369 817          
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1,101                  
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 $ 2,154 2,140 2,187 2,201 2,199          
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 $ 2,074 2,029 1,921 1,760 1,307          
Short-duration Insurance Contract, Cumulative Number of Reported Claims 135                  
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 $ 1,253 1,337 1,327 1,318 1,248          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 333                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 704 591 462 287 112          
Short-duration Insurance Contract, Cumulative Number of Reported Claims 44                  
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 $ 1,814 1,852 1,879 1,938 1,868          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 29                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,724 1,693 1,630 1,421 755          
Short-duration Insurance Contract, Cumulative Number of Reported Claims 591                  
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 $ 341 339 336 331 330          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 36                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 263 247 216 183 91          
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 180 181 178 178 162          
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 $ 167 161 151 128 $ 64          
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 1,279 1,276 1,259 1,282            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 594                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 501 418 301 116            
Short-duration Insurance Contract, Cumulative Number of Reported Claims 334                  
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 $ 3,968 3,812 3,702 3,553            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1,570                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,853 1,204 604 138            
Short-duration Insurance Contract, Cumulative Number of Reported Claims 21                  
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 $ 514 501 469 486            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 148                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 304 214 137 47            
Short-duration Insurance Contract, Cumulative Number of Reported Claims 15                  
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 $ 1,634 1,646 1,741 1,732            
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 $ 1,553 1,485 1,340 725            
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1,170                  
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 $ 2,538 2,555 2,544 2,489            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 48                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,386 2,266 2,079 1,495            
Short-duration Insurance Contract, Cumulative Number of Reported Claims 139                  
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 $ 1,314 1,293 1,267 1,170            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 387                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 659 482 282 86            
Short-duration Insurance Contract, Cumulative Number of Reported Claims 46                  
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 $ 2,036 2,071 2,117 1,992            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 63                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,853 1,772 1,548 850            
Short-duration Insurance Contract, Cumulative Number of Reported Claims 621                  
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 $ 297 296 286 281            
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 $ 215 190 157 89            
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 160 160 153 145            
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 $ 142 132 103 $ 56            
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 1,383 1,361 1,366              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 743                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 452 326 122              
Short-duration Insurance Contract, Cumulative Number of Reported Claims 304                  
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 $ 3,686 3,589 3,528              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 1,726                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,335 662 171              
Short-duration Insurance Contract, Cumulative Number of Reported Claims 21                  
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 $ 527 501 503              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 196                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 246 145 52              
Short-duration Insurance Contract, Cumulative Number of Reported Claims 15                  
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 $ 1,795 1,885 1,906              
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 $ 1,651 1,500 844              
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1,291                  
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 $ 2,539 2,530 2,434              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 190                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,206 2,047 1,450              
Short-duration Insurance Contract, Cumulative Number of Reported Claims 140                  
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 $ 1,345 1,278 1,179              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 555                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 521 316 123              
Short-duration Insurance Contract, Cumulative Number of Reported Claims 46                  
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Overseas General Insurance - Non-Casualty [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net $ 1,998 2,009 2,032              
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 $ 1,851 1,654 1,016              
Short-duration Insurance Contract, Cumulative Number of Reported Claims 619                  
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 $ 231 223 219              
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 $ 141 111 57              
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 187 185 179              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 13                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 157 130 $ 56              
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 1,380 1,412                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 831                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 313 120                
Short-duration Insurance Contract, Cumulative Number of Reported Claims 338                  
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 $ 3,492 3,317                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 2,442                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 616 161                
Short-duration Insurance Contract, Cumulative Number of Reported Claims 21                  
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 $ 565 531                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 265                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 175 66                
Short-duration Insurance Contract, Cumulative Number of Reported Claims 16                  
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 $ 2,604 2,700                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 207                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,084 977                
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1,372                  
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 $ 3,064 3,029                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 291                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 2,515 1,694                
Short-duration Insurance Contract, Cumulative Number of Reported Claims 144                  
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 $ 1,287 1,186                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 676                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 314 96                
Short-duration Insurance Contract, Cumulative Number of Reported Claims 44                  
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 $ 2,236 2,199                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 29                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,822 1,040                
Short-duration Insurance Contract, Cumulative Number of Reported Claims 659                  
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 $ 211 210                
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 $ 99 46                
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 422 396                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 32                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 322 $ 191                
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 1,359                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 995                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 130                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 321                  
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 $ 3,369                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 2,950                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 189                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 24                  
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 $ 535                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 383                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 74                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 14                  
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 $ 2,048                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 474                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 1,026                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1,326                  
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 $ 3,003                  
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 $ 1,923                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 129                  
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 $ 1,287                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 989                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 109                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 32                  
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 $ 2,161                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 437                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 987                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 627                  
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 $ 239                  
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 $ 40                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0                  
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 $ 283                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net 93                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 94                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 0