Reserve for Property and Casualty Insurance Claims and Claims Expense - Claims and allocated claim adjustment expenses, net of reinsurance (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) claim	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Increase (decrease) in claims and claims expense
Liabilities for claims and claim adjustment expenses, net of recoverables	$ 20,577
Auto insurance - liability coverage
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	42,687
Prior year reserve reestimates	7
Prior year reserve reestimates	7
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	31,287
All outstanding liabilities before 2016, net of recoverables	1,389
Liabilities for claims and claim adjustment expenses, net of recoverables	12,789
Auto insurance - liability coverage | Accident Year 2016
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	8,709	$ 8,690	$ 8,740	$ 8,841	$ 9,038
Prior year reserve reestimates	19
IBNR reserves plus expected development on reported claims	$ 554
Cumulative number of reported claims | claim	2,400,904
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 8,155	7,700	6,853	5,772	3,487
Auto insurance - liability coverage | Accident Year 2017
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	8,330	8,312	8,396	8,465
Prior year reserve reestimates	18
IBNR reserves plus expected development on reported claims	$ 1,017
Cumulative number of reported claims | claim	2,217,132
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 7,313	6,532	5,333	3,151
Auto insurance - liability coverage | Accident Year 2018
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	8,731	8,715	8,734
Prior year reserve reestimates	16
IBNR reserves plus expected development on reported claims	$ 1,846
Cumulative number of reported claims | claim	2,180,275
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 6,885	5,618	3,231
Auto insurance - liability coverage | Accident Year 2019
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	9,295	9,341
Prior year reserve reestimates	(46)
IBNR reserves plus expected development on reported claims	$ 3,064
Cumulative number of reported claims | claim	2,205,813
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 6,231	3,498
Auto insurance - liability coverage | Accident Year 2020
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	7,622
IBNR reserves plus expected development on reported claims	$ 4,919
Cumulative number of reported claims | claim	1,500,921
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 2,703
Auto insurance - liability coverage | Prior year reserve reestimates for pre-2016 accident years
Increase (decrease) in claims and claims expense
Prior year reserve reestimates	(20)
Auto insurance - liability coverage | Prior year reserve reestimates for ULAE
Increase (decrease) in claims and claims expense
Prior year reserve reestimates	23
Auto insurance - physical damage coverage
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	25,687
Prior year reserve reestimates	(107)
Prior year reserve reestimates	(114)
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	25,322
All outstanding liabilities before 2016, net of recoverables	8
Liabilities for claims and claim adjustment expenses, net of recoverables	373
Auto insurance - physical damage coverage | Accident Year 2016
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	5,022	5,023	5,027	5,054	5,128
Prior year reserve reestimates	(1)
IBNR reserves plus expected development on reported claims	$ 5
Cumulative number of reported claims | claim	4,432,048
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 5,017	5,018	5,022	5,033	4,890
Auto insurance - physical damage coverage | Accident Year 2017
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	5,027	5,028	5,039	5,121
Prior year reserve reestimates	(1)
IBNR reserves plus expected development on reported claims	$ 1
Cumulative number of reported claims | claim	4,237,772
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 5,026	5,030	5,039	4,847
Auto insurance - physical damage coverage | Accident Year 2018
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	5,108	5,157	5,219
Prior year reserve reestimates	(49)
IBNR reserves plus expected development on reported claims	$ 9
Cumulative number of reported claims | claim	4,310,750
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 5,099	5,140	4,971
Auto insurance - physical damage coverage | Accident Year 2019
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	5,606	5,662
Prior year reserve reestimates	(56)
IBNR reserves plus expected development on reported claims	$ 26
Cumulative number of reported claims | claim	4,469,473
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 5,580	5,418
Auto insurance - physical damage coverage | Accident Year 2020
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	4,924
IBNR reserves plus expected development on reported claims	$ 324
Cumulative number of reported claims | claim	3,493,530
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 4,600
Auto insurance - physical damage coverage | Prior year reserve reestimates for pre-2016 accident years
Increase (decrease) in claims and claims expense
Prior year reserve reestimates	(1)
Auto insurance - physical damage coverage | Prior year reserve reestimates for ULAE
Increase (decrease) in claims and claims expense
Prior year reserve reestimates	(5)
Homeowners
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	22,859
Prior year reserve reestimates	(480)
Prior year reserve reestimates	(439)
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	20,874
All outstanding liabilities before 2016, net of recoverables	119
Liabilities for claims and claim adjustment expenses, net of recoverables	2,104
Homeowners | Accident Year 2016
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	3,948	3,953	3,958	3,995	3,961
Prior year reserve reestimates	(5)
IBNR reserves plus expected development on reported claims	$ 43
Cumulative number of reported claims | claim	814,026
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 3,905	3,876	3,811	3,680	$ 2,949
Homeowners | Accident Year 2017
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	4,390	4,614	4,619	4,477
Prior year reserve reestimates	(224)
IBNR reserves plus expected development on reported claims	$ 61
Cumulative number of reported claims | claim	908,714
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 4,329	4,437	4,249	$ 3,228
Homeowners | Accident Year 2018
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	4,551	4,854	4,749
Prior year reserve reestimates	(303)
IBNR reserves plus expected development on reported claims	$ 182
Cumulative number of reported claims | claim	811,046
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 4,369	4,514	$ 3,491
Homeowners | Accident Year 2019
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	4,601	4,549
Prior year reserve reestimates	52
IBNR reserves plus expected development on reported claims	$ 301
Cumulative number of reported claims | claim	784,257
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 4,300	$ 3,316
Homeowners | Accident Year 2020
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	5,369
IBNR reserves plus expected development on reported claims	$ 1,398
Cumulative number of reported claims | claim	838,829
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 3,971
Homeowners | Prior year reserve reestimates for pre-2016 accident years
Increase (decrease) in claims and claims expense
Prior year reserve reestimates	(11)
Homeowners | Prior year reserve reestimates for ULAE
Increase (decrease) in claims and claims expense
Prior year reserve reestimates	52
Other | Auto insurance - liability coverage
Increase (decrease) in claims and claims expense
Prior year reserve reestimates	(3)
Other | Auto insurance - physical damage coverage
Increase (decrease) in claims and claims expense
Prior year reserve reestimates	(1)
Other | Homeowners
Increase (decrease) in claims and claims expense
Prior year reserve reestimates	$ 0