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Reserve for Unpaid Losses and Loss Adjustment Expenses - Historical Loss Development Triangles, P&C (Details)
$ in Millions
Dec. 31, 2024
USD ($)
claim
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Workers' Compensation | Segment Reconciling Items                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 18,389                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 9,665                  
Workers' Compensation | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,625 $ 1,625 $ 1,645 $ 1,667 $ 1,699 $ 1,714 $ 1,724 $ 1,801 $ 1,835 $ 1,873
IBNR Reserves $ 329                  
Claims Reported | claim 114,700                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,200 1,179 1,151 1,117 1,068 1,004 909 778 576 261
Workers' Compensation | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,621 1,634 1,670 1,708 1,748 1,767 1,780 1,772 1,772  
IBNR Reserves $ 331                  
Claims Reported | claim 112,747                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,173 1,145 1,110 1,064 1,003 908 779 579 255  
Workers' Compensation | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,597 1,635 1,665 1,757 1,822 1,840 1,869 1,862    
IBNR Reserves $ 392                  
Claims Reported | claim 112,274                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,118 1,087 1,035 977 900 778 575 261    
Workers' Compensation | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,798 1,836 1,870 1,904 1,915 1,917 1,916      
IBNR Reserves $ 464                  
Claims Reported | claim 120,076                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,215 1,170 1,090 983 837 624 283      
Workers' Compensation | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,864 1,899 1,934 1,934 1,935 1,937        
IBNR Reserves $ 528                  
Claims Reported | claim 121,158                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,204 1,129 1,007 856 637 291        
Workers' Compensation | 2020                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,712 1,808 1,849 1,864 1,865          
IBNR Reserves $ 601                  
Claims Reported | claim 92,347                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 939 850 695 507 223          
Workers' Compensation | Short-Duration Insurance Contract, Accident Year 2021 [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,831 1,831 1,832 1,831            
IBNR Reserves $ 675                  
Claims Reported | claim 103,229                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 920 780 562 254            
Workers' Compensation | Short-Duration Insurance Contract, Accident Year 2022                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 2,001 2,001 2,000              
IBNR Reserves $ 819                  
Claims Reported | claim 114,189                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 910 649 293              
Workers' Compensation | Short-Duration Insurance Contract, Accident Year 2023                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 2,166 2,166                
IBNR Reserves $ 1,171                  
Claims Reported | claim 117,297                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 677 286                
Workers' Compensation | Short-Duration Insurance Contract, Accident Year 2024                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 2,174                  
IBNR Reserves $ 1,536                  
Claims Reported | claim 109,521                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 309                  
General liability | Segment Reconciling Items                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 9,416                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 4,177                  
General liability | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 656 641 647 637 647 633 594 554 560 556
IBNR Reserves $ 60                  
Claims Reported | claim 16,903                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 569 564 547 524 477 409 278 156 55 10
General liability | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 692 670 636 620 632 632 607 583 613  
IBNR Reserves $ 68                  
Claims Reported | claim 17,989                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 596 564 513 446 368 283 131 52 12  
General liability | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 690 657 615 613 615 613 614 626    
IBNR Reserves $ 82                  
Claims Reported | claim 17,581                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 553 506 441 344 255 156 67 15    
General liability | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 816 751 728 703 697 669 692      
IBNR Reserves $ 167                  
Claims Reported | claim 19,019                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 595 512 409 288 177 83 21      
General liability | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 876 859 839 821 826 822        
IBNR Reserves $ 190                  
Claims Reported | claim 18,871                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 613 501 339 192 100 29        
General liability | 2020                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 857 873 922 922 938          
IBNR Reserves $ 363                  
Claims Reported | claim 14,574                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 432 308 202 110 45          
General liability | Short-Duration Insurance Contract, Accident Year 2021 [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,000 983 991 1,002            
IBNR Reserves $ 493                  
Claims Reported | claim 12,839                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 394 209 115 34            
General liability | Short-Duration Insurance Contract, Accident Year 2022                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,167 1,110 1,116              
IBNR Reserves $ 746                  
Claims Reported | claim 12,673                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 281 134 26              
General liability | Short-Duration Insurance Contract, Accident Year 2023                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,230 1,219                
IBNR Reserves $ 989                  
Claims Reported | claim 10,854                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 128 17                
General liability | Short-Duration Insurance Contract, Accident Year 2024                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,432                  
IBNR Reserves $ 1,366                  
Claims Reported | claim 7,290                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 16                  
Marine [Member] | Segment Reconciling Items                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,427                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,114                  
Marine [Member] | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 139 140 142 139 138 133 148 146 146 158
IBNR Reserves $ 1                  
Claims Reported | claim 10,559                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 141 142 142 140 139 133 125 116 85 40
Marine [Member] | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 157 158 148 146 149 147 137 142 139  
IBNR Reserves $ 0                  
Claims Reported | claim 13,811                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 141 146 143 140 131 122 105 80 35  
Marine [Member] | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 172 174 167 165 159 160 173 153    
IBNR Reserves $ 0                  
Claims Reported | claim 16,235                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 167 170 161 150 143 133 106 47    
Marine [Member] | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 155 156 152 146 141 146 131      
IBNR Reserves $ (5)                  
Claims Reported | claim 10,812                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 151 159 142 135 126 94 33      
Marine [Member] | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 127 127 129 134 135 139        
IBNR Reserves $ (3)                  
Claims Reported | claim 7,261                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 120 115 105 96 80 34        
Marine [Member] | 2020                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 142 137 134 137 145          
IBNR Reserves $ 7                  
Claims Reported | claim 5,261                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 114 99 90 68 31          
Marine [Member] | Short-Duration Insurance Contract, Accident Year 2021 [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 127 119 127 127            
IBNR Reserves $ 15                  
Claims Reported | claim 5,357                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 101 87 63 25            
Marine [Member] | Short-Duration Insurance Contract, Accident Year 2022                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 130 132 140              
IBNR Reserves $ 22                  
Claims Reported | claim 5,306                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 89 72 27              
Marine [Member] | Short-Duration Insurance Contract, Accident Year 2023                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 129 134                
IBNR Reserves $ 39                  
Claims Reported | claim 4,674                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 58 22                
Marine [Member] | Short-Duration Insurance Contract, Accident Year 2024                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 149                  
IBNR Reserves $ 83                  
Claims Reported | claim 3,735                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 32                  
Package Business [Member] | Segment Reconciling Items                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 8,630                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 6,279                  
Package Business [Member] | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 570 564 564 567 581 588 583 585 588 582
IBNR Reserves $ 17                  
Claims Reported | claim 42,479                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 549 542 530 513 505 486 445 383 332 212
Package Business [Member] | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 582 590 591 595 611 625 632 638 655  
IBNR Reserves $ 24                  
Claims Reported | claim 44,381                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 549 545 540 521 500 465 410 353 225  
Package Business [Member] | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 638 640 637 644 657 692 702 695    
IBNR Reserves $ 33                  
Claims Reported | claim 46,958                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 593 578 561 534 496 447 372 235    
Package Business [Member] | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 671 654 655 667 688 724 719      
IBNR Reserves $ 46                  
Claims Reported | claim 45,361                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 609 571 537 498 451 402 237      
Package Business [Member] | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 761 747 744 749 769 813        
IBNR Reserves $ 72                  
Claims Reported | claim 43,955                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 666 626 571 488 413 254        
Package Business [Member] | 2020                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 828 837 877 893 915          
IBNR Reserves $ 101                  
Claims Reported | claim 62,964                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 699 648 573 493 326          
Package Business [Member] | Short-Duration Insurance Contract, Accident Year 2021 [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 958 958 954 946            
IBNR Reserves $ 150                  
Claims Reported | claim 47,755                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 746 650 556 368            
Package Business [Member] | Short-Duration Insurance Contract, Accident Year 2022                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,043 1,039 1,038              
IBNR Reserves $ 215                  
Claims Reported | claim 47,134                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 728 633 319              
Package Business [Member] | Short-Duration Insurance Contract, Accident Year 2023                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,223 1,250                
IBNR Reserves $ 378                  
Claims Reported | claim 46,364                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 725 453                
Package Business [Member] | Short-Duration Insurance Contract, Accident Year 2024                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,356                  
IBNR Reserves $ 750                  
Claims Reported | claim 42,172                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 415                  
Property Insurance [Member] | P&C Business Insurance [Member] | Segment Reconciling Items                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 4,244                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 3,676                  
Property Insurance [Member] | P&C Business Insurance [Member] | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 299 301 301 301 304 305 301 302 301 298
IBNR Reserves $ 0                  
Claims Reported | claim 20,977                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 300 301 302 302 303 301 296 284 257 179
Property Insurance [Member] | P&C Business Insurance [Member] | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 405 405 405 408 408 406 399 419 405  
IBNR Reserves $ 0                  
Claims Reported | claim 23,927                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 406 407 406 406 401 395 378 342 215  
Property Insurance [Member] | P&C Business Insurance [Member] | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 438 439 438 440 438 456 516 577    
IBNR Reserves $ 2                  
Claims Reported | claim 24,627                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 441 440 438 432 427 411 378 229    
Property Insurance [Member] | P&C Business Insurance [Member] | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 393 393 400 403 423 436 450      
IBNR Reserves $ 2                  
Claims Reported | claim 21,821                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 394 394 394 385 378 344 188      
Property Insurance [Member] | P&C Business Insurance [Member] | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 420 421 420 418 439 480        
IBNR Reserves $ (1)                  
Claims Reported | claim 20,981                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 410 407 405 383 351 215        
Property Insurance [Member] | P&C Business Insurance [Member] | 2020                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 437 438 440 469 501          
IBNR Reserves $ 49                  
Claims Reported | claim 20,529                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 373 367 355 336 221          
Property Insurance [Member] | P&C Business Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2021 [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 434 463 500 531            
IBNR Reserves $ 15                  
Claims Reported | claim 18,283                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 412 403 383 241            
Property Insurance [Member] | P&C Business Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2022                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 475 481 497              
IBNR Reserves $ 48                  
Claims Reported | claim 17,446                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 412 369 180              
Property Insurance [Member] | P&C Business Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2023                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 424 448                
IBNR Reserves $ 86                  
Claims Reported | claim 17,003                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 300 199                
Property Insurance [Member] | P&C Business Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2024                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 519                  
IBNR Reserves $ 164                  
Claims Reported | claim 15,209                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 228                  
Property Insurance [Member] | P&C Personal Insurance [Member] | Segment Reconciling Items                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 5,876                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 5,521                  
Property Insurance [Member] | P&C Personal Insurance [Member] | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 682 682 684 684 684 684 684 690 703 690
IBNR Reserves $ 0                  
Claims Reported | claim 120,023                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 682 682 682 681 681 680 674 665 645 487
Property Insurance [Member] | P&C Personal Insurance [Member] | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 658 658 658 658 658 658 663 673 669  
IBNR Reserves $ 0                  
Claims Reported | claim 119,818                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 658 657 656 655 653 649 640 621 481  
Property Insurance [Member] | P&C Personal Insurance [Member] | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 769 771 774 775 783 884 889 866    
IBNR Reserves $ 4                  
Claims Reported | claim 124,780                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 763 761 762 761 757 795 747 538    
Property Insurance [Member] | P&C Personal Insurance [Member] | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 642 645 639 642 673 910 903      
IBNR Reserves $ 8                  
Claims Reported | claim 102,924                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 628 626 627 619 616 712 484      
Property Insurance [Member] | P&C Personal Insurance [Member] | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 465 467 468 470 475 501        
IBNR Reserves $ 1                  
Claims Reported | claim 84,810                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 463 460 458 445 425 318        
Property Insurance [Member] | P&C Personal Insurance [Member] | 2020                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 499 505 513 512 525          
IBNR Reserves $ 5                  
Claims Reported | claim 88,546                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 490 486 478 454 335          
Property Insurance [Member] | P&C Personal Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2021 [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 485 491 501 502            
IBNR Reserves $ 5                  
Claims Reported | claim 77,320                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 473 464 440 305            
Property Insurance [Member] | P&C Personal Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2022                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 498 507 499              
IBNR Reserves $ 9                  
Claims Reported | claim 64,118                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 476 453 298              
Property Insurance [Member] | P&C Personal Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2023                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 573 584                
IBNR Reserves $ 28                  
Claims Reported | claim 68,405                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 521 390                
Property Insurance [Member] | P&C Personal Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2024                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 605                  
IBNR Reserves $ 158                  
Claims Reported | claim 58,223                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 367                  
Automobile liability | P&C Business Insurance [Member] | Segment Reconciling Items                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 4,606                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 3,106                  
Automobile liability | P&C Business Insurance [Member] | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 358 360 358 360 359 356 356 372 358 308
IBNR Reserves $ 5                  
Claims Reported | claim 28,750                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 351 350 348 344 335 314 267 207 142 62
Automobile liability | P&C Business Insurance [Member] | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 395 396 395 395 391 391 390 393 385  
IBNR Reserves $ 5                  
Claims Reported | claim 29,266                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 388 385 379 357 339 303 232 147 65  
Automobile liability | P&C Business Insurance [Member] | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 398 398 394 381 383 379 383 372    
IBNR Reserves $ 2                  
Claims Reported | claim 26,420                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 389 386 368 328 285 211 134 60    
Automobile liability | P&C Business Insurance [Member] | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 435 433 424 406 405 396 349      
IBNR Reserves $ 18                  
Claims Reported | claim 24,816                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 406 387 360 305 238 153 62      
Automobile liability | P&C Business Insurance [Member] | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 479 471 460 450 439 425        
IBNR Reserves $ 20                  
Claims Reported | claim 28,627                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 428 393 327 247 160 67        
Automobile liability | P&C Business Insurance [Member] | 2020                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 388 397 419 424 428          
IBNR Reserves $ 44                  
Claims Reported | claim 22,272                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 317 264 200 119 55          
Automobile liability | P&C Business Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2021 [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 410 429 443 440            
IBNR Reserves $ 86                  
Claims Reported | claim 20,222                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 282 212 127 55            
Automobile liability | P&C Business Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2022                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 547 500 468              
IBNR Reserves $ 158                  
Claims Reported | claim 20,891                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 294 171 64              
Automobile liability | P&C Business Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2023                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 555 527                
IBNR Reserves $ 279                  
Claims Reported | claim 20,649                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 174 69                
Automobile liability | P&C Business Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2024                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 641                  
IBNR Reserves $ 501                  
Claims Reported | claim 19,954                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 77                  
Automobile liability | P&C Personal Insurance [Member] | Segment Reconciling Items                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 10,862                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 9,240                  
Automobile liability | P&C Personal Insurance [Member] | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,319 1,319 1,320 1,324 1,328 1,331 1,330 1,338 1,340 1,195
IBNR Reserves $ 2                  
Claims Reported | claim 216,911                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,315 1,314 1,313 1,310 1,304 1,292 1,243 1,142 935 475
Automobile liability | P&C Personal Insurance [Member] | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,388 1,384 1,384 1,386 1,395 1,397 1,393 1,402 1,407  
IBNR Reserves $ 6                  
Claims Reported | claim 215,877                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,380 1,377 1,373 1,363 1,345 1,308 1,188 968 505  
Automobile liability | P&C Personal Insurance [Member] | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,198 1,197 1,198 1,200 1,214 1,228 1,275 1,277    
IBNR Reserves $ 6                  
Claims Reported | claim 187,564                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,189 1,187 1,180 1,161 1,123 1,033 836 441    
Automobile liability | P&C Personal Insurance [Member] | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,054 1,055 1,056 1,058 1,072 1,104 1,108      
IBNR Reserves $ 18                  
Claims Reported | claim 156,295                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 1,033 1,028 1,011 965 888 710 359      
Automobile liability | P&C Personal Insurance [Member] | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 967 971 986 991 1,010 1,018        
IBNR Reserves $ 10                  
Claims Reported | claim 139,762                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 949 933 897 816 654 323        
Automobile liability | P&C Personal Insurance [Member] | 2020                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 740 741 775 782 805          
IBNR Reserves $ 23                  
Claims Reported | claim 96,731                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 709 679 615 486 238          
Automobile liability | P&C Personal Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2021 [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 846 852 886 881            
IBNR Reserves $ 57                  
Claims Reported | claim 102,129                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 760 691 553 247            
Automobile liability | P&C Personal Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2022                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,009 1,018 928              
IBNR Reserves $ 132                  
Claims Reported | claim 107,911                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 813 662 301              
Automobile liability | P&C Personal Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2023                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,129 1,138                
IBNR Reserves $ 270                  
Claims Reported | claim 107,253                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 731 329                
Automobile liability | P&C Personal Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2024                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,212                  
IBNR Reserves $ 622                  
Claims Reported | claim 94,233                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 361                  
Commercial automobile physical damage | P&C Business Insurance [Member] | Segment Reconciling Items                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 239                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 213                  
Commercial automobile physical damage | P&C Business Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2022                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 74 74 70              
IBNR Reserves $ 2                  
Claims Reported | claim 16,771                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 72 73 59              
Commercial automobile physical damage | P&C Business Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2023                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 81 80                
IBNR Reserves $ 6                  
Claims Reported | claim 16,832                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 74 61                
Commercial automobile physical damage | P&C Business Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2024                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 84                  
IBNR Reserves $ 8                  
Claims Reported | claim 15,533                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 67                  
Professional liability | Segment Reconciling Items                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 3,042                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,567                  
Professional liability | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 192 197 196 198 206 213 190 179 174 164
IBNR Reserves $ 1                  
Claims Reported | claim 7,279                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 181 185 175 163 140 124 107 85 40 9
Professional liability | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 202 191 193 195 194 196 203 176 183  
IBNR Reserves $ 20                  
Claims Reported | claim 8,462                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 175 176 166 147 124 111 88 51 8  
Professional liability | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 227 217 242 239 225 231 203 205    
IBNR Reserves $ 14                  
Claims Reported | claim 9,555                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 189 190 179 149 121 87 48 11    
Professional liability | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 312 324 265 271 270 274 243      
IBNR Reserves $ 18                  
Claims Reported | claim 9,910                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 260 232 194 161 127 71 15      
Professional liability | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 385 354 347 330 313 295        
IBNR Reserves $ 81                  
Claims Reported | claim 9,972                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 265 241 197 147 77 21        
Professional liability | 2020                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 298 324 336 363 369          
IBNR Reserves $ 106                  
Claims Reported | claim 8,034                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 171 147 117 71 19          
Professional liability | Short-Duration Insurance Contract, Accident Year 2021 [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 306 327 343 339            
IBNR Reserves $ 149                  
Claims Reported | claim 6,761                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 127 95 55 15            
Professional liability | Short-Duration Insurance Contract, Accident Year 2022                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 338 355 349              
IBNR Reserves $ 214                  
Claims Reported | claim 7,177                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 95 64 18              
Professional liability | Short-Duration Insurance Contract, Accident Year 2023                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 388 384                
IBNR Reserves $ 260                  
Claims Reported | claim 8,049                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 76 20                
Professional liability | Short-Duration Insurance Contract, Accident Year 2024                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 394                  
IBNR Reserves $ 337                  
Claims Reported | claim 7,546                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 28                  
Surety Product Line [Member] | Segment Reconciling Items                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 667                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 276                  
Surety Product Line [Member] | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 29 37 42 47 48 54 60 63 67 67
IBNR Reserves $ 4                  
Claims Reported | claim 1,413                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 25 25 25 30 32 34 31 24 20 9
Surety Product Line [Member] | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 28 34 37 45 51 55 61 61 61  
IBNR Reserves $ 8                  
Claims Reported | claim 1,348                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 20 20 22 22 22 20 15 12 2  
Surety Product Line [Member] | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 65 68 70 79 94 101 90 63    
IBNR Reserves $ 22                  
Claims Reported | claim 1,799                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 43 43 43 42 54 55 46 5    
Surety Product Line [Member] | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 54 63 70 71 72 68 68      
IBNR Reserves $ 24                  
Claims Reported | claim 1,750                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 29 30 29 24 23 16 6      
Surety Product Line [Member] | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 61 70 73 74 73 72        
IBNR Reserves $ 44                  
Claims Reported | claim 1,925                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 17 16 16 15 13 3        
Surety Product Line [Member] | 2020                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 80 83 79 84 83          
IBNR Reserves $ 51                  
Claims Reported | claim 2,294                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 27 26 21 12 4          
Surety Product Line [Member] | Short-Duration Insurance Contract, Accident Year 2021 [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 84 88 85 85            
IBNR Reserves $ 55                  
Claims Reported | claim 2,989                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 29 23 21 8            
Surety Product Line [Member] | Short-Duration Insurance Contract, Accident Year 2022                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 93 93 85              
IBNR Reserves $ 29                  
Claims Reported | claim 2,571                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 59 42 11              
Surety Product Line [Member] | Short-Duration Insurance Contract, Accident Year 2023                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 83 81                
IBNR Reserves $ 63                  
Claims Reported | claim 1,647                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 17 8                
Surety Product Line [Member] | Short-Duration Insurance Contract, Accident Year 2024                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 90                  
IBNR Reserves $ 78                  
Claims Reported | claim 975                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 10                  
Assumed Reinsurance [Member] | Segment Reconciling Items                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 2,107                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,356                  
Assumed Reinsurance [Member] | 2015                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 96 96 96 96 96 95 94 94 92 102
IBNR Reserves $ 0                  
Claims Reported | claim 1,737                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 96 96 95 95 94 91 83 77 64 $ 42
Assumed Reinsurance [Member] | 2016                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 104 104 102 101 102 100 98 91 88  
IBNR Reserves $ (1)                  
Claims Reported | claim 1,996                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 101 101 99 97 95 90 84 66 $ 36  
Assumed Reinsurance [Member] | 2017                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 155 155 155 153 157 161 153 129    
IBNR Reserves $ 0                  
Claims Reported | claim 2,596                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 150 151 149 147 145 135 116 $ 44    
Assumed Reinsurance [Member] | 2018                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 132 132 135 134 129 127 128      
IBNR Reserves $ (6)                  
Claims Reported | claim 3,078                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 133 144 141 139 133 111 $ 25      
Assumed Reinsurance [Member] | 2019                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 208 208 190 186 189 181        
IBNR Reserves $ 11                  
Claims Reported | claim 3,843                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 185 176 159 153 132 $ 62        
Assumed Reinsurance [Member] | 2020                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 181 178 186 180 183          
IBNR Reserves $ 11                  
Claims Reported | claim 3,374                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 151 132 113 89 $ 49          
Assumed Reinsurance [Member] | Short-Duration Insurance Contract, Accident Year 2021 [Member]                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 204 203 195 191            
IBNR Reserves $ 22                  
Claims Reported | claim 2,698                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 157 132 102 $ 46            
Assumed Reinsurance [Member] | Short-Duration Insurance Contract, Accident Year 2022                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 290 274 266              
IBNR Reserves $ 84                  
Claims Reported | claim 2,501                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 173 128 60              
Assumed Reinsurance [Member] | Short-Duration Insurance Contract, Accident Year 2023                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 327 329                
IBNR Reserves $ 112                  
Claims Reported | claim 2,354                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 149 63                
Assumed Reinsurance [Member] | Short-Duration Insurance Contract, Accident Year 2024                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 410                  
IBNR Reserves $ 297                  
Claims Reported | claim 1,049                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 61                  
Personal automobile physical damage | P&C Personal Insurance [Member] | Segment Reconciling Items                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,640                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 1,574                  
Personal automobile physical damage | P&C Personal Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2022                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 539 549 533              
IBNR Reserves $ 3                  
Claims Reported | claim 239,634                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 536 538 $ 498              
Personal automobile physical damage | P&C Personal Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2023                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 544 574                
IBNR Reserves $ 3                  
Claims Reported | claim 234,247                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 541 $ 513                
Personal automobile physical damage | P&C Personal Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2024                    
Claims Development [Line Items]                    
Incurred Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance 557                  
IBNR Reserves $ 28                  
Claims Reported | claim 199,915                  
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance $ 497