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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, 2017
USD ($)
claim
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Dec. 31, 2012
USD ($)
Dec. 31, 2011
USD ($)
Dec. 31, 2010
USD ($)
Dec. 31, 2009
USD ($)
Dec. 31, 2008
USD ($)
Workers’ compensation                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 18,351                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 10,945                  
Workers’ compensation | 2008                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,493 $ 0 $ 1,492 $ 1,477 $ 1,477 $ 1,473 $ 1,470 $ 1,456 $ 1,444 $ 1,456
IBNR Reserves $ 123                  
Claims Reported | claim 141,627                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,242 1,216 1,190 1,146 1,089 1,015 917 781 581 264
Workers’ compensation | 2009                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,522 0 1,519 1,504 1,504 1,493 1,478 1,455 1,462  
IBNR Reserves $ 166                  
Claims Reported | claim 135,731                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,242 1,208 1,170 1,115 1,042 937 792 587 265  
Workers’ compensation | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,878 0 1,882 1,857 1,858 1,814 1,775 1,560    
IBNR Reserves $ 242                  
Claims Reported | claim 156,515                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 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 2,232 0 2,221 2,206 2,204 2,099 2,013      
IBNR Reserves $ 361                  
Claims Reported | claim 177,652                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 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 2,169 0 2,181 2,207 2,207 2,185        
IBNR Reserves $ 443                  
Claims Reported | claim 171,021                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 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 1,861 0 1,920 1,981 2,020          
IBNR Reserves $ 510                  
Claims Reported | claim 150,884                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,175 1,071 917 675 304          
Workers’ compensation | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,761 0 1,838 1,869            
IBNR Reserves $ 638                  
Claims Reported | claim 125,487                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 960 811 598 275            
Workers’ compensation | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,801 0 1,873              
IBNR Reserves $ 806                  
Claims Reported | claim 112,970                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 778 576 261              
Workers’ compensation | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,772 0                
IBNR Reserves $ 944                  
Claims Reported | claim 110,072                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 579 255                
Workers’ compensation | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,862                  
IBNR Reserves $ 1,349                  
Claims Reported | claim 102,626                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 261                  
Liability                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 3,473                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,894                  
Liability | 2008                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 394 398 401 398 416 451 454 468 457 501
IBNR Reserves $ 40                  
Claims Reported | claim 21,374                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 343 337 330 318 300 270 216 141 69 31
Liability | 2009                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 341 346 347 348 359 382 394 398 382  
IBNR Reserves $ 33                  
Claims Reported | claim 20,530                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 297 287 277 256 227 181 124 63 22  
Liability | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 377 376 345 343 355 352 362 355    
IBNR Reserves $ 33                  
Claims Reported | claim 18,729                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 331 314 259 224 181 115 51 14    
Liability | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 318 320 315 316 323 343 353      
IBNR Reserves $ 49                  
Claims Reported | claim 16,637                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 252 234 198 154 93 47 11      
Liability | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 298 304 295 310 315 321        
IBNR Reserves $ 66                  
Claims Reported | claim 11,614                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 198 167 124 75 39 8        
Liability | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 344 352 332 321 318          
IBNR Reserves $ 88                  
Claims Reported | claim 9,715                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 207 152 95 35 7          
Liability | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 342 336 318 317            
IBNR Reserves $ 128                  
Claims Reported | claim 10,048                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 142 88 31 11            
Liability | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 345 346 316              
IBNR Reserves $ 194                  
Claims Reported | claim 10,326                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 80 32 7              
Liability | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 351 352                
IBNR Reserves $ 262                  
Claims Reported | claim 11,028                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 32 8                
Liability | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 363                  
IBNR Reserves $ 302                  
Claims Reported | claim 8,823                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 12                  
Package business                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 6,553                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 5,198                  
Package business | 2008                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 674 675 673 676 674 675 677 709 703 667
IBNR Reserves $ 16                  
Claims Reported | claim 58,109                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 652 0 643 633 620 595 562 510 451 278
Package business | 2009                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 574 576 576 577 578 572 584 584 587  
IBNR Reserves $ 20                  
Claims Reported | claim 50,351                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 550 0 539 527 503 463 411 351 227  
Package business | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 651 653 651 652 652 654 662 657    
IBNR Reserves $ 24                  
Claims Reported | claim 52,345                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 618 0 601 570 539 487 414 270    
Package business | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 813 814 808 800 790 792 810      
IBNR Reserves $ 37                  
Claims Reported | claim 60,892                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 762 0 727 684 621 555 377      
Package business | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 735 736 731 728 725 736        
IBNR Reserves $ 38                  
Claims Reported | claim 59,621                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 673 0 616 560 486 286        
Package business | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 586 585 573 565 579          
IBNR Reserves $ 50                  
Claims Reported | claim 43,284                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 504 0 414 339 225          
Package business | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 602 601 578 566            
IBNR Reserves $ 86                  
Claims Reported | claim 42,718                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 468 0 345 226            
Package business | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 585 588 582              
IBNR Reserves $ 137                  
Claims Reported | claim 41,202                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 383 0 212              
Package business | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 638 655                
IBNR Reserves $ 228                  
Claims Reported | claim 42,042                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 353 0                
Package business | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 695                  
IBNR Reserves $ 348                  
Claims Reported | claim 39,524                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 235                  
Commercial property                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 3,263                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 2,898                  
Commercial property | 2008                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 464 464 463 464 464 467 464 465 465 478
IBNR Reserves $ 0                  
Claims Reported | claim 31,995                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 465 466 465 464 464 464 459 449 422 280
Commercial property | 2009                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 257 257 257 257 251 258 259 264 267  
IBNR Reserves $ 0                  
Claims Reported | claim 28,286                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 257 257 257 257 256 256 252 247 179  
Commercial property | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 284 284 284 284 282 279 283 286    
IBNR Reserves $ 0                  
Claims Reported | claim 28,515                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 284 284 284 283 281 276 266 198    
Commercial property | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 359 360 361 362 356 356 357      
IBNR Reserves $ 0                  
Claims Reported | claim 29,103                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 360 359 358 355 350 332 231      
Commercial property | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 305 306 305 301 301 329        
IBNR Reserves $ 1                  
Claims Reported | claim 25,785                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 303 304 300 294 279 171        
Commercial property | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 216 220 219 218 234          
IBNR Reserves $ 0                  
Claims Reported | claim 20,287                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 215 218 216 208 157          
Commercial property | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 264 262 260 268            
IBNR Reserves $ 0                  
Claims Reported | claim 19,742                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 264 258 243 168            
Commercial property | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 268 264 264              
IBNR Reserves $ 3                  
Claims Reported | claim 19,031                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 255 239 172              
Commercial property | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 331 328                
IBNR Reserves $ 6                  
Claims Reported | claim 19,868                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 285 188                
Commercial property | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 515                  
IBNR Reserves $ 81                  
Claims Reported | claim 18,883                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 210                  
Commercial automobile liability                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 3,446                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 2,559                  
Commercial automobile liability | 2008                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 306 306 307 302 304 304 303 304 311 303
IBNR Reserves $ 4                  
Claims Reported | claim 43,885                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 302 300 299 295 289 270 238 185 124 61
Commercial automobile liability | 2009                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 302 302 302 301 297 287 287 292 306  
IBNR Reserves $ 1                  
Claims Reported | claim 38,688                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 301 300 298 291 274 237 175 115 56  
Commercial automobile liability | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 324 327 328 323 319 296 280 277    
IBNR Reserves $ 7                  
Claims Reported | claim 38,112                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 313 308 300 289 252 188 125 55    
Commercial automobile liability | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 362 365 366 356 356 310 272      
IBNR Reserves $ 7                  
Claims Reported | claim 39,262                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 348 339 315 273 211 133 62      
Commercial automobile liability | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 390 394 401 390 376 311        
IBNR Reserves $ 14                  
Claims Reported | claim 35,970                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 358 345 306 233 142 65        
Commercial automobile liability | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 335 336 329 314 309          
IBNR Reserves $ 24                  
Claims Reported | claim 31,881                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 289 255 199 128 61          
Commercial automobile liability | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 333 328 314 306            
IBNR Reserves $ 48                  
Claims Reported | claim 29,171                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 249 195 129 58            
Commercial automobile liability | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 368 353 302              
IBNR Reserves $ 99                  
Claims Reported | claim 27,928                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 204 141 61              
Commercial automobile liability | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 380 372                
IBNR Reserves $ 155                  
Claims Reported | claim 27,771                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 140 62                
Commercial automobile liability | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 346                  
IBNR Reserves $ 243                  
Claims Reported | claim 22,665                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 55                  
Commercial automobile physical damage                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 238                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 227                  
Commercial automobile physical damage | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 75 75 74              
IBNR Reserves $ 0                  
Claims Reported | claim 26,812                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 75 75 69              
Commercial automobile physical damage | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 78 79                
IBNR Reserves $ 0                  
Claims Reported | claim 26,320                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 78 71                
Commercial automobile physical damage | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 85                  
IBNR Reserves $ 4                  
Claims Reported | claim 22,965                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 74                  
Professional liability                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,647                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,146                  
Professional liability | 2008                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 284 277 282 276 276 280 274 244 253 281
IBNR Reserves $ 16                  
Claims Reported | claim 4,959                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 266 264 260 230 221 202 166 126 61 13
Professional liability | 2009                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 257 257 255 263 257 266 244 251 254  
IBNR Reserves $ 19                  
Claims Reported | claim 5,114                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 235 226 225 226 194 177 127 69 17  
Professional liability | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 199 195 200 201 205 212 211 202    
IBNR Reserves $ 31                  
Claims Reported | claim 4,890                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 166 162 157 148 137 103 62 22    
Professional liability | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 220 219 219 226 232 228 226      
IBNR Reserves $ 44                  
Claims Reported | claim 4,707                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 173 170 163 128 100 57 11      
Professional liability | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 144 146 149 168 172 174        
IBNR Reserves $ 33                  
Claims Reported | claim 3,729                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 107 97 89 60 41 11        
Professional liability | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 103 110 123 136 136          
IBNR Reserves $ 35                  
Claims Reported | claim 2,782                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 55 39 31 19 4          
Professional liability | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 114 118 123 116            
IBNR Reserves $ 44                  
Claims Reported | claim 2,878                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 64 40 21 4            
Professional liability | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 113 113 104              
IBNR Reserves $ 51                  
Claims Reported | claim 2,943                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 49 23 4              
Professional liability | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 106 106                
IBNR Reserves $ 66                  
Claims Reported | claim 3,090                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 25 4                
Professional liability | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 107                  
IBNR Reserves $ 82                  
Claims Reported | claim 2,733                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 6                  
Bond                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 598                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 333                  
Bond | 2008                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 46 48 47 44 47 47 52 62 67 75
IBNR Reserves $ 7                  
Claims Reported | claim 3,450                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 39 39 39 39 34 32 30 23 18 5
Bond | 2009                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 49 49 49 51 57 58 69 71 71  
IBNR Reserves $ 4                  
Claims Reported | claim 3,309                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 44 44 44 43 44 46 45 32 9  
Bond | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 90 70 69 73 79 80 75 71    
IBNR Reserves $ 24                  
Claims Reported | claim 2,670                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 66 66 63 59 58 59 46 13    
Bond | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 69 70 70 75 76 76 72      
IBNR Reserves $ 10                  
Claims Reported | claim 2,126                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 59 59 57 56 51 39 12      
Bond | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 48 48 53 60 69 69        
IBNR Reserves $ 16                  
Claims Reported | claim 1,719                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 25 25 24 26 25 12        
Bond | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 48 48 54 58 63          
IBNR Reserves $ 29                  
Claims Reported | claim 1,452                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 18 18 17 9 3          
Bond | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 66 65 65 69            
IBNR Reserves $ 20                  
Claims Reported | claim 1,362                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 43 40 31 18            
Bond | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 62 65 65              
IBNR Reserves $ 29                  
Claims Reported | claim 1,347                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 23 19 9              
Bond | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 59 59                
IBNR Reserves $ 45                  
Claims Reported | claim 1,227                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 11 2                
Bond | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 61                  
IBNR Reserves $ 53                  
Claims Reported | claim 1,018                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 5                  
Personal automobile liability                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 12,363                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 10,749                  
Personal automobile liability | 2008                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,188 1,188 1,191 1,192 1,196 1,197 1,207 1,227 1,249 1,253
IBNR Reserves $ 3                  
Claims Reported | claim 248,990                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,184 1,184 1,183 1,181 1,175 1,160 1,121 1,031 861 469
Personal automobile liability | 2009                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,257 1,257 1,259 1,260 1,256 1,255 1,280 1,305 1,351  
IBNR Reserves $ 3                  
Claims Reported | claim 254,551                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,251 1,250 1,246 1,240 1,223 1,171 1,083 888 492  
Personal automobile liability | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,265 1,265 1,275 1,282 1,287 1,293 1,321 1,346    
IBNR Reserves $ 3                  
Claims Reported | claim 248,944                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,258 1,256 1,251 1,239 1,202 1,108 915 496    
Personal automobile liability | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,154 1,154 1,166 1,173 1,180 1,170 1,181      
IBNR Reserves $ 7                  
Claims Reported | claim 221,879                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,145 1,140 1,126 1,088 1,006 826 447      
Personal automobile liability | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,130 1,133 1,142 1,146 1,149 1,141        
IBNR Reserves $ 8                  
Claims Reported | claim 210,740                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,114 1,104 1,067 986 818 441        
Personal automobile liability | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,152 1,153 1,144 1,145 1,131          
IBNR Reserves $ 14                  
Claims Reported | claim 205,428                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,121 1,091 1,002 816 442          
Personal automobile liability | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,200 1,198 1,153 1,146            
IBNR Reserves $ 34                  
Claims Reported | claim 208,817                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,125 1,032 843 430            
Personal automobile liability | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,338 1,340 1,195              
IBNR Reserves $ 92                  
Claims Reported | claim 216,189                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,142 935 475              
Personal automobile liability | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,402 1,407                
IBNR Reserves $ 235                  
Claims Reported | claim 213,563                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 968 505                
Personal automobile liability | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,277                  
IBNR Reserves $ 554                  
Claims Reported | claim 175,871                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 441                  
Personal automobile physical damage                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,884                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,856                  
Personal automobile physical damage | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 630 632 629              
IBNR Reserves $ 0                  
Claims Reported | claim 395,923                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 629 630 610              
Personal automobile physical damage | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 656 665                
IBNR Reserves $ 2                  
Claims Reported | claim 406,162                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 653 634                
Personal automobile physical damage | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 598                  
IBNR Reserves $ (8)                  
Claims Reported | claim 343,178                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 574                  
Homeowners                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 7,588                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 7,161                  
Homeowners | 2008                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 779 780 779 779 779 779 778 777 768 742
IBNR Reserves $ 0                  
Claims Reported | claim 165,108                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 778 778 777 777 775 773 764 750 721 $ 548
Homeowners | 2009                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 768 769 772 772 772 772 776 777 757  
IBNR Reserves $ 0                  
Claims Reported | claim 149,790                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 767 766 766 765 763 759 749 727 $ 559  
Homeowners | 2010                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 834 836 840 840 840 838 850 838    
IBNR Reserves $ 0                  
Claims Reported | claim 161,581                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 833 833 832 829 825 815 789 $ 599    
Homeowners | 2011                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 908 911 914 916 919 920 955      
IBNR Reserves $ 1                  
Claims Reported | claim 179,377                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 908 905 903 899 891 871 $ 709      
Homeowners | 2012                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 738 739 741 741 741 774        
IBNR Reserves $ 2                  
Claims Reported | claim 142,804                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 734 731 727 719 696 $ 547        
Homeowners | 2013                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 632 634 637 638 673          
IBNR Reserves $ 4                  
Claims Reported | claim 113,469                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 626 622 611 590 $ 467          
Homeowners | 2014                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 700 702 707 710            
IBNR Reserves $ 5                  
Claims Reported | claim 121,809                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 691 684 663 $ 526            
Homeowners | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 690 703 690              
IBNR Reserves $ 9                  
Claims Reported | claim 119,722                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 665 645 $ 487              
Homeowners | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 673 669                
IBNR Reserves $ 20                  
Claims Reported | claim 118,748                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 621 $ 481                
Homeowners | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 866                  
IBNR Reserves $ 96                  
Claims Reported | claim 115,488                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 538