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Reserve for Unpaid Losses and Loss Adjustment Expenses - Historical Loss Development Triangles, P&C (Details)
claim in Millions, $ in Millions
Dec. 31, 2019
USD ($)
claim
Dec. 31, 2018
USD ($)
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 ($)
Dec. 31, 2010
USD ($)
Workers’ compensation                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 18,990                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 10,991                  
Workers’ compensation | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,888 $ 1,892 $ 1,878 $ 1,881 $ 1,882 $ 1,857 $ 1,858 $ 1,814 $ 1,775 $ 1,560
IBNR Reserves $ 221                  
Claims Reported | claim 156,802                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,553 1,522 1,489 1,439 1,374 1,287 1,154 970 709 316
Workers’ compensation | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 2,239 2,242 2,232 2,224 2,221 2,206 2,204 2,099 2,013  
IBNR Reserves $ 314                  
Claims Reported | claim 177,910                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,786 1,746 1,690 1,622 1,518 1,368 1,156 841 371  
Workers’ compensation | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 2,146 2,154 2,169 2,168 2,181 2,207 2,207 2,185    
IBNR Reserves $ 350                  
Claims Reported | claim 171,341                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,644 1,587 1,529 1,436 1,313 1,106 809 359    
Workers’ compensation | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,850 1,861 1,861 1,883 1,920 1,981 2,020      
IBNR Reserves $ 415                  
Claims Reported | claim 151,315                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,304 1,260 1,175 1,071 917 675 304      
Workers’ compensation | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,692 1,713 1,761 1,789 1,838 1,869        
IBNR Reserves $ 477                  
Claims Reported | claim 126,104                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,099 1,041 960 811 598 275        
Workers’ compensation | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,714 1,724 1,801 1,835 1,873          
IBNR Reserves $ 540                  
Claims Reported | claim 113,819                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,004 909 778 576 261          
Workers’ compensation | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,767 1,780 1,772 1,772            
IBNR Reserves $ 665                  
Claims Reported | claim 111,763                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 908 779 579 255            
Workers’ compensation | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,840 1,869 1,862              
IBNR Reserves $ 864                  
Claims Reported | claim 111,096                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 778 575 261              
Workers’ compensation | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,917 1,916                
IBNR Reserves $ 1,039                  
Claims Reported | claim 116,915                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 624 283                
Workers’ compensation | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,937                  
IBNR Reserves $ 1,359                  
Claims Reported | claim 110,515                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 291                  
General liability                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 5,711                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 2,828                  
General liability | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 482 483 467 465 431 428 437 432 445 436
IBNR Reserves $ 41                  
Claims Reported | claim 23,941                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 427 418 409 387 327 284 230 149 68 20
General liability | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 420 426 417 416 404 405 408 420 431  
IBNR Reserves $ 48                  
Claims Reported | claim 22,310                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 362 348 330 303 255 200 123 61 15  
General liability | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 413 421 408 410 392 399 402 423    
IBNR Reserves $ 65                  
Claims Reported | claim 16,501                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 323 305 280 233 170 101 55 13    
General liability | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 505 502 488 484 456 442 455      
IBNR Reserves $ 77                  
Claims Reported | claim 13,643                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 398 372 320 233 141 53 13      
General liability | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 522 513 494 481 475 506        
IBNR Reserves $ 114                  
Claims Reported | claim 14,318                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 358 304 214 130 42 15        
General liability | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 633 594 554 560 556          
IBNR Reserves $ 141                  
Claims Reported | claim 15,088                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 409 278 156 55 10          
General liability | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 633 607 583 613            
IBNR Reserves $ 254                  
Claims Reported | claim 15,984                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 283 131 52 12            
General liability | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 613 614 626              
IBNR Reserves $ 359                  
Claims Reported | claim 15,039                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 156 67 15              
General liability | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 669 692                
IBNR Reserves $ 516                  
Claims Reported | claim 15,368                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 83 21                
General liability | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 821                  
IBNR Reserves $ 744                  
Claims Reported | claim 11,628                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 29                  
Marine [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,226                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 974                  
Marine [Member] | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 163 167 163 163 168 179 219 195    
IBNR Reserves $ 0                  
Claims Reported | claim 6,766                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 158 154 152 148 139 125 101 50    
Marine [Member] | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 137 134 139 135 134 152 148      
IBNR Reserves $ (2)                  
Claims Reported | claim 6,601                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 125 120 118 111 100 82 41      
Marine [Member] | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 168 163 164 157 159 163        
IBNR Reserves $ (1)                  
Claims Reported | claim 7,093                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 156 150 130 116 80 40        
Marine [Member] | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 133 148 145 145 158          
IBNR Reserves $ (8)                  
Claims Reported | claim 10,038                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 133 125 115 85 40          
Marine [Member] | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 147 137 142 139            
IBNR Reserves $ (3)                  
Claims Reported | claim 12,959                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 122 106 80 35            
Marine [Member] | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 174 186 160              
IBNR Reserves $ 3                  
Claims Reported | claim 15,216                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 141 110 48              
Marine [Member] | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 160 144                
IBNR Reserves $ 5                  
Claims Reported | claim 13,130                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 104 37                
Marine [Member] | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 144                  
IBNR Reserves $ 61                  
Claims Reported | claim 5,775                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 35                  
Package business                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 6,817                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 5,215                  
Package business | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 647 649 651 653 651 652 652 654 662 657
IBNR Reserves $ 18                  
Claims Reported | claim 52,484                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 627 625 618 613 601 570 539 487 414 270
Package business | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 807 812 813 814 808 800 790 792 810  
IBNR Reserves $ 26                  
Claims Reported | claim 61,045                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 774 772 762 748 727 684 621 555 377  
Package business | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 732 739 735 736 731 728 725 736    
IBNR Reserves $ 30                  
Claims Reported | claim 59,817                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 694 687 673 652 616 560 486 286    
Package business | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 586 592 586 585 573 565 579      
IBNR Reserves $ 32                  
Claims Reported | claim 43,556                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 541 522 504 467 414 339 225      
Package business | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 603 603 602 601 578 566        
IBNR Reserves $ 59                  
Claims Reported | claim 43,098                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 525 507 468 416 345 226        
Package business | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 588 583 585 588 582          
IBNR Reserves $ 69                  
Claims Reported | claim 41,965                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 486 445 383 332 212          
Package business | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 625 632 638 655            
IBNR Reserves $ 118                  
Claims Reported | claim 43,672                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 465 410 353 225            
Package business | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 692 702 695              
IBNR Reserves $ 192                  
Claims Reported | claim 45,836                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 447 372 235              
Package business | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 724 719                
IBNR Reserves $ 241                  
Claims Reported | claim 43,026                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 402 237                
Package business | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 813                  
IBNR Reserves $ 392                  
Claims Reported | claim 36,824                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 254                  
Property | Property and Casualty, Commercial Insurance [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 2,939                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 2,520                  
Property | Property and Casualty, Commercial Insurance [Member] | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 333 334 335 336 334 334 333 369    
IBNR Reserves $ 1                  
Claims Reported | claim 26,786                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 330 331 331 331 326 317 296 182    
Property | Property and Casualty, Commercial Insurance [Member] | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 247 248 249 252 253 252 268      
IBNR Reserves $ 0                  
Claims Reported | claim 21,601                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 245 244 242 243 238 223 161      
Property | Property and Casualty, Commercial Insurance [Member] | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 281 279 280 282 281 293        
IBNR Reserves $ 0                  
Claims Reported | claim 21,017                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 279 279 279 270 250 170        
Property | Property and Casualty, Commercial Insurance [Member] | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 305 301 302 301 298          
IBNR Reserves $ 1                  
Claims Reported | claim 21,005                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 301 296 284 257 179          
Property | Property and Casualty, Commercial Insurance [Member] | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 406 399 419 405            
IBNR Reserves $ 1                  
Claims Reported | claim 23,710                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 395 378 342 215            
Property | Property and Casualty, Commercial Insurance [Member] | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 455 515 577              
IBNR Reserves $ 21                  
Claims Reported | claim 24,235                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 412 378 229              
Property | Property and Casualty, Commercial Insurance [Member] | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 436 450                
IBNR Reserves $ 38                  
Claims Reported | claim 21,460                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 344 188                
Property | Property and Casualty, Commercial Insurance [Member] | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 476                  
IBNR Reserves $ 93                  
Claims Reported | claim 18,634                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 214                  
Automobile liability | Property and Casualty, Commercial Insurance [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 3,698                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 2,739                  
Automobile liability | Property and Casualty, Commercial Insurance [Member] | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 339 340 341 344 344 338 335 309 291 290
IBNR Reserves $ 3                  
Claims Reported | claim 38,158                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 335 334 330 324 315 305 266 199 132 60
Automobile liability | Property and Casualty, Commercial Insurance [Member] | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 363 362 363 365 366 356 356 310 272  
IBNR Reserves $ 6                  
Claims Reported | claim 39,298                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 354 353 348 339 316 274 211 133 63  
Automobile liability | Property and Casualty, Commercial Insurance [Member] | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 388 387 389 395 402 391 377 311    
IBNR Reserves $ 6                  
Claims Reported | claim 36,043                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 376 372 359 346 307 234 143 65    
Automobile liability | Property and Casualty, Commercial Insurance [Member] | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 335 339 340 341 334 318 311      
IBNR Reserves $ 11                  
Claims Reported | claim 32,228                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 321 311 295 259 202 130 62      
Automobile liability | Property and Casualty, Commercial Insurance [Member] | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 334 341 337 331 317 309        
IBNR Reserves $ 14                  
Claims Reported | claim 29,597                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 309 299 252 197 131 59        
Automobile liability | Property and Casualty, Commercial Insurance [Member] | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 356 356 372 358 308          
IBNR Reserves $ 18                  
Claims Reported | claim 28,487                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 314 267 207 142 62          
Automobile liability | Property and Casualty, Commercial Insurance [Member] | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 391 390 393 385            
IBNR Reserves $ 44                  
Claims Reported | claim 29,036                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 303 232 147 65            
Automobile liability | Property and Casualty, Commercial Insurance [Member] | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 379 383 372              
IBNR Reserves $ 76                  
Claims Reported | claim 26,089                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 211 134 60              
Automobile liability | Property and Casualty, Commercial Insurance [Member] | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 396 349                
IBNR Reserves $ 153                  
Claims Reported | claim 24,016                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 153 62                
Automobile liability | Property and Casualty, Commercial Insurance [Member] | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 417                  
IBNR Reserves $ 291                  
Claims Reported | claim 22,455                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 63                  
Automobile liability | Property and Casualty, Personal Insurance [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 11,985                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 10,518                  
Automobile liability | Property and Casualty, Personal Insurance [Member] | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,265 1,264 1,265 1,265 1,275 1,282 1,287 1,293 1,321 1,346
IBNR Reserves $ 3                  
Claims Reported | claim 248,948                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,260 1,260 1,258 1,256 1,251 1,239 1,202 1,108 915 496
Automobile liability | Property and Casualty, Personal Insurance [Member] | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,153 1,153 1,154 1,154 1,166 1,173 1,180 1,170 1,181  
IBNR Reserves $ 4                  
Claims Reported | claim 221,890                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,146 1,146 1,145 1,140 1,126 1,088 1,006 826 447  
Automobile liability | Property and Casualty, Personal Insurance [Member] | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,130 1,130 1,130 1,133 1,142 1,146 1,149 1,141    
IBNR Reserves $ 6                  
Claims Reported | claim 210,757                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,122 1,120 1,114 1,104 1,067 986 818 441    
Automobile liability | Property and Casualty, Personal Insurance [Member] | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,157 1,153 1,152 1,153 1,144 1,145 1,131      
IBNR Reserves $ 8                  
Claims Reported | claim 205,475                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,142 1,135 1,121 1,091 1,002 816 442      
Automobile liability | Property and Casualty, Personal Insurance [Member] | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,202 1,199 1,200 1,198 1,153 1,146        
IBNR Reserves $ 11                  
Claims Reported | claim 208,983                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,182 1,165 1,125 1,032 843 430        
Automobile liability | Property and Casualty, Personal Insurance [Member] | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,331 1,330 1,338 1,340 1,195          
IBNR Reserves $ 21                  
Claims Reported | claim 216,827                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,292 1,243 1,142 935 475          
Automobile liability | Property and Casualty, Personal Insurance [Member] | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,397 1,393 1,402 1,407            
IBNR Reserves $ 46                  
Claims Reported | claim 215,658                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,308 1,188 968 505            
Automobile liability | Property and Casualty, Personal Insurance [Member] | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,228 1,275 1,277              
IBNR Reserves $ 109                  
Claims Reported | claim 186,993                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,033 836 441              
Automobile liability | Property and Casualty, Personal Insurance [Member] | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,104 1,108                
IBNR Reserves $ 246                  
Claims Reported | claim 154,648                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 710 359                
Automobile liability | Property and Casualty, Personal Insurance [Member] | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,018                  
IBNR Reserves $ 461                  
Claims Reported | claim 131,577                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 323                  
Commercial automobile physical damage                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 206                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 194                  
Commercial automobile physical damage | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 81 81 85              
IBNR Reserves $ 3                  
Claims Reported | claim 24,325                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 78 79 74              
Commercial automobile physical damage | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 62 62                
IBNR Reserves $ 1                  
Claims Reported | claim 20,508                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 60 54                
Commercial automobile physical damage | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 63                  
IBNR Reserves $ 2                  
Claims Reported | claim 18,626                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 56                  
Professional liability | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 225 219 220 221 218 238 238 242    
IBNR Reserves $ 19                  
Claims Reported | claim 7,025                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 175 172 168 154 139 100 67 17    
Professional liability | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 171 173 173 174 187 195 207      
IBNR Reserves $ 24                  
Claims Reported | claim 5,970                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 137 131 116 88 67 44 10      
Professional liability | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 182 179 177 181 183 187        
IBNR Reserves $ 26                  
Claims Reported | claim 6,705                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 135 130 107 74 38 7        
Professional liability | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 213 190 179 174 164          
IBNR Reserves $ 51                  
Claims Reported | claim 7,171                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 124 107 85 40 9          
Professional liability | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 196 203 176 183            
IBNR Reserves $ 66                  
Claims Reported | claim 8,288                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 111 88 51 8            
Professional liability | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 231 203 205              
IBNR Reserves $ 103                  
Claims Reported | claim 9,224                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 87 48 11              
Professional liability | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 280 247                
IBNR Reserves $ 155                  
Claims Reported | claim 9,517                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 73 15                
Professional liability | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 298                  
IBNR Reserves $ 252                  
Claims Reported | claim 7,396                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 21                  
Bond                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 637                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 353                  
Bond | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 72 73 92 72 71 75 81 82 76 72
IBNR Reserves $ 6                  
Claims Reported | claim 2,674                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 64 69 67 67 65 60 60 60 46 14
Bond | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 72 71 71 71 71 76 78 78 74  
IBNR Reserves $ 9                  
Claims Reported | claim 2,136                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 61 60 60 60 58 57 52 40 12  
Bond | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 48 45 49 49 55 61 70 71    
IBNR Reserves $ 13                  
Claims Reported | claim 1,723                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 35 34 26 26 24 26 25 12    
Bond | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 35 39 49 48 55 58 64      
IBNR Reserves $ 15                  
Claims Reported | claim 1,463                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 20 19 19 19 17 9 3      
Bond | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 59 59 67 66 67 71        
IBNR Reserves $ 12                  
Claims Reported | claim 1,383                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 45 43 43 40 31 18        
Bond | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 54 60 63 67 67          
IBNR Reserves $ 19                  
Claims Reported | claim 1,385                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 34 31 24 20 9          
Bond | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 56 61 61 61            
IBNR Reserves $ 32                  
Claims Reported | claim 1,324                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 20 15 12 2            
Bond | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 101 90 63              
IBNR Reserves $ 42                  
Claims Reported | claim 1,547                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 55 46 5              
Bond | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 68 68                
IBNR Reserves $ 49                  
Claims Reported | claim 1,383                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 16 6                
Bond | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 72                  
IBNR Reserves $ 68                  
Claims Reported | claim 1,122                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 3                  
Assumed Reinsurance [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,005                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 824                  
Assumed Reinsurance [Member] | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 121 119 120 115 88 93 99 107    
IBNR Reserves $ 0                  
Claims Reported | claim 1,424                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 119 118 118 112 85 83 77 38    
Assumed Reinsurance [Member] | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 104 102 102 105 103 119 115      
IBNR Reserves $ 1                  
Claims Reported | claim 1,607                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 103 101 100 98 91 83 53      
Assumed Reinsurance [Member] | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 116 115 118 122 142 119        
IBNR Reserves $ 1                  
Claims Reported | claim 1,654                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 113 112 109 106 119 66        
Assumed Reinsurance [Member] | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 95 94 94 92 102          
IBNR Reserves $ 0                  
Claims Reported | claim 1,383                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 91 83 77 64 42          
Assumed Reinsurance [Member] | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 100 98 91 88            
IBNR Reserves $ 3                  
Claims Reported | claim 1,434                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 90 84 66 36            
Assumed Reinsurance [Member] | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 161 153 129              
IBNR Reserves $ 11                  
Claims Reported | claim 1,582                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 135 116 44              
Assumed Reinsurance [Member] | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 127 128                
IBNR Reserves $ 1                  
Claims Reported | claim 1,322                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 111 25                
Assumed Reinsurance [Member] | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 181                  
IBNR Reserves $ 107                  
Claims Reported | claim 875                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 62                  
Personal automobile physical damage                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,531                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,507                  
Personal automobile physical damage | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 588 588 598              
IBNR Reserves $ (1)                  
Claims Reported | claim 362,235                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 589 591 574              
Personal automobile physical damage | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 498 509                
IBNR Reserves $ 7                  
Claims Reported | claim 305,031                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 491 474                
Personal automobile physical damage | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 445                  
IBNR Reserves $ (11)                  
Claims Reported | claim 262,866                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 427                  
Homeowners                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 7,443                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 6,958                  
Homeowners | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 834 834 834 836 840 840 840 838 850 838
IBNR Reserves $ (1)                  
Claims Reported | claim 161,597                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 835 834 833 833 832 829 825 815 789 $ 599
Homeowners | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 907 907 908 911 914 916 919 920 955  
IBNR Reserves $ 0                  
Claims Reported | claim 179,399                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 908 907 908 905 903 899 891 871 $ 709  
Homeowners | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 738 738 738 739 741 741 741 774    
IBNR Reserves $ 1                  
Claims Reported | claim 142,845                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 736 735 734 731 727 719 696 $ 547    
Homeowners | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 629 630 632 634 637 638 673      
IBNR Reserves $ 1                  
Claims Reported | claim 113,538                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 628 627 626 622 611 590 $ 467      
Homeowners | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 698 698 700 702 707 710        
IBNR Reserves $ (1)                  
Claims Reported | claim 121,902                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 697 695 691 684 663 $ 526        
Homeowners | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 684 684 690 703 690          
IBNR Reserves $ 2                  
Claims Reported | claim 119,944                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 680 674 665 645 $ 487          
Homeowners | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 658 663 673 669            
IBNR Reserves $ 4                  
Claims Reported | claim 119,646                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 649 640 621 $ 481            
Homeowners | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 884 889 866              
IBNR Reserves $ 41                  
Claims Reported | claim 124,189                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 795 747 $ 538              
Homeowners | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 910 903                
IBNR Reserves $ 60                  
Claims Reported | claim 101,985                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 712 $ 484                
Homeowners | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 501                  
IBNR Reserves $ 107                  
Claims Reported | claim 78,068                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 318