Reserve for Property and Casualty Insurance Claims and Claims Expense - Schedule of Incurred Claims, Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Recoverables (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) claim	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Auto insurance - liability coverage
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	$ 59,524
Prior year reserve reestimates	387
Total prior year reserve reestimates	622
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	41,368
All outstanding liabilities before 2019, net of recoverables	1,368
Liabilities for claims and claim adjustment expenses, net of recoverables	19,524
Auto insurance - liability coverage | Accident Year 2019
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	10,812	$ 10,729	$ 10,557	$ 10,307	$ 10,377
Prior year reserve reestimates	83
IBNR reserves plus expected development on reported claims	$ 642
Cumulative number of reported claims | claim	2,569,785
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 10,170	9,519	8,530	6,995	3,936
Auto insurance - liability coverage | Accident Year 2020
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	8,904	8,871	8,631	8,651	0
Prior year reserve reestimates	33
IBNR reserves plus expected development on reported claims	$ 997
Cumulative number of reported claims | claim	1,898,342
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 7,907	7,089	5,796	3,111	0
Auto insurance - liability coverage | Accident Year 2021
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	11,180	10,851	10,365	0	0
Prior year reserve reestimates	329
IBNR reserves plus expected development on reported claims	$ 2,057
Cumulative number of reported claims | claim	2,229,946
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 9,123	7,304	3,589	0	0
Auto insurance - liability coverage | Accident Year 2022
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	13,263	13,321	0	0	0
Prior year reserve reestimates	(58)
IBNR reserves plus expected development on reported claims	$ 4,202
Cumulative number of reported claims | claim	2,401,908
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 9,061	4,424	0	0	0
Auto insurance - liability coverage | Accident Year 2023
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	15,365	0	0	0	0
IBNR reserves plus expected development on reported claims	$ 10,258
Cumulative number of reported claims | claim	2,283,467
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 5,107	0	0	0	0
Auto insurance - liability coverage | Prior year reserve reestimates for pre-2019 accident years
Increase (decrease) in claims and claims expense
Total prior year reserve reestimates	229
Auto insurance - liability coverage | Prior year reserve reestimates for ULAE
Increase (decrease) in claims and claims expense
Total prior year reserve reestimates	6
Auto insurance - liability coverage | Other
Increase (decrease) in claims and claims expense
Total prior year reserve reestimates	0
Auto insurance - physical damage coverage
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	37,229
Prior year reserve reestimates	(356)
Total prior year reserve reestimates	(378)
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	36,385
All outstanding liabilities before 2019, net of recoverables	0
Liabilities for claims and claim adjustment expenses, net of recoverables	844
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	6,113	6,120	6,121	6,162	6,241
Prior year reserve reestimates	(7)
IBNR reserves plus expected development on reported claims	$ (11)
Cumulative number of reported claims | claim	4,864,602
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 6,124	6,123	6,126	6,132	5,936
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	5,359	5,369	5,393	5,484	0
Prior year reserve reestimates	(10)
IBNR reserves plus expected development on reported claims	$ (11)
Cumulative number of reported claims | claim	3,989,262
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 5,370	5,378	5,405	5,119	0
Auto insurance - physical damage coverage | Accident Year 2021
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	7,269	7,326	7,269	0	0
Prior year reserve reestimates	(57)
IBNR reserves plus expected development on reported claims	$ (25)
Cumulative number of reported claims | claim	4,567,859
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 7,294	7,351	6,847	0	0
Auto insurance - physical damage coverage | Accident Year 2022
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	9,004	9,286	0	0	0
Prior year reserve reestimates	(282)
IBNR reserves plus expected development on reported claims	$ (79)
Cumulative number of reported claims | claim	4,904,706
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 9,083	8,243	0	0	0
Auto insurance - physical damage coverage | Accident Year 2023
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	9,484	0	0	0	0
IBNR reserves plus expected development on reported claims	$ 970
Cumulative number of reported claims | claim	4,591,024
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 8,514	0	0	0	0
Auto insurance - physical damage coverage | Prior year reserve reestimates for pre-2019 accident years
Increase (decrease) in claims and claims expense
Total prior year reserve reestimates	(5)
Auto insurance - physical damage coverage | Prior year reserve reestimates for ULAE
Increase (decrease) in claims and claims expense
Total prior year reserve reestimates	(20)
Auto insurance - physical damage coverage | Other
Increase (decrease) in claims and claims expense
Total prior year reserve reestimates	3
Homeowners
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	32,829
Prior year reserve reestimates	72
Total prior year reserve reestimates	102
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	28,474
All outstanding liabilities before 2019, net of recoverables	95
Liabilities for claims and claim adjustment expenses, net of recoverables	4,450
Homeowners | Accident Year 2019
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	4,790	4,781	4,769	4,762	4,694
Prior year reserve reestimates	9
IBNR reserves plus expected development on reported claims	$ 38
Cumulative number of reported claims | claim	847,160
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 4,752	4,699	4,621	4,432	3,414
Homeowners | Accident Year 2020
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	5,824	5,773	5,680	5,626	0
Prior year reserve reestimates	51
IBNR reserves plus expected development on reported claims	$ 83
Cumulative number of reported claims | claim	977,647
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 5,741	5,649	5,423	4,162	0
Homeowners | Accident Year 2021
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	6,468	6,408	6,282	0	0
Prior year reserve reestimates	60
IBNR reserves plus expected development on reported claims	$ 198
Cumulative number of reported claims | claim	989,192
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 6,270	6,035	4,449	0	0
Homeowners | Accident Year 2022
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	6,489	6,537	0	0	0
Prior year reserve reestimates	(48)
IBNR reserves plus expected development on reported claims	$ 567
Cumulative number of reported claims | claim	784,867
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 5,922	3,866	0	0	0
Homeowners | Accident Year 2023
Increase (decrease) in claims and claims expense
Incurred claims and allocated claim adjustment expenses, net of recoverables	9,258	0	0	0	0
IBNR reserves plus expected development on reported claims	$ 3,469
Cumulative number of reported claims | claim	919,333
Cumulative paid claims and allocated claims adjustment expenses, net of recoverables	$ 5,789	$ 0	$ 0	$ 0	$ 0
Homeowners | Prior year reserve reestimates for pre-2019 accident years
Increase (decrease) in claims and claims expense
Total prior year reserve reestimates	29
Homeowners | Prior year reserve reestimates for ULAE
Increase (decrease) in claims and claims expense
Total prior year reserve reestimates	(17)
Homeowners | Other
Increase (decrease) in claims and claims expense
Total prior year reserve reestimates	$ 18