Claim and Claim Adjustment Expense Reserves - Workers' Compensation (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 ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2014	$ 3,679	$ 3,481	$ 3,016
Total net liability for unpaid claim and claim adjustment expenses	18,163	16,929	16,300	$ 15,857
Property and casualty
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total net liability for unpaid claim and claim adjustment expenses	17,213	15,996
Workers’ compensation
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	4,172
IBNR	1,238
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	2,418
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,754
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2014	1,842
Other	(23)
Liability for unallocated claim adjustment expenses for accident years presented	72
Total net liability for unpaid claim and claim adjustment expenses	3,645
Net Strengthening (Releases) of Prior Accident Year Reserves
Total net development for the accident years presented above	(144)	(99)	(46)
Adjustment for development on a discounted basis	(2)	(3)	2
Total net development for accident years prior to 2014	(63)	(60)	(38)
Total unallocated claim adjustment expense development	6	10	0
Total	(203)	(152)	(82)
Workers’ compensation | Short-duration Insurance Contracts, Accident Year 2014
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	419	430	448	439	$ 446	$ 450	$ 452	$ 479	$ 480	$ 467
IBNR	$ 70
Cumulative number of claims | Claim	33,550
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 319	312	306	297	290	282	258	215	159	$ 61
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(11)	(18)	9	(7)	(4)	(2)	(27)	(1)	13
IBNR, Total	(48)
Workers’ compensation | Short-duration Insurance Contracts, Accident Year 2015
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	334	353	372	382	394	408	406	431	422
IBNR	$ 59
Cumulative number of claims | Claim	31,904
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 259	256	251	243	231	212	180	131	$ 51
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(19)	(19)	(10)	(12)	(14)	2	(25)	9
IBNR, Total	(88)
Workers’ compensation | Short-duration Insurance Contracts, Accident Year 2016
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	308	331	355	366	382	396	405	426
IBNR	$ 56
Cumulative number of claims | Claim	31,994
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 235	234	227	219	198	169	129	$ 53
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(23)	(24)	(11)	(16)	(14)	(9)	(21)
IBNR, Total	(118)
Workers’ compensation | Short-duration Insurance Contracts, Accident Year 2017
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	398	399	402	400	421	432	440
IBNR	$ 78
Cumulative number of claims | Claim	33,142
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 287	279	265	243	207	151	$ 63
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(1)	(3)	2	(21)	(11)	(8)
IBNR, Total	(42)
Workers’ compensation | Short-duration Insurance Contracts, Accident Year 2018
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	404	415	415	428	440	450
IBNR	$ 74
Cumulative number of claims | Claim	34,886
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 298	280	259	229	163	$ 68
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(11)	0	(13)	(12)	(10)
IBNR, Total	(46)
Workers’ compensation | Short-Duration Insurance Contract, Accident Year 2019
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	419	436	437	449	452
IBNR	$ 78
Cumulative number of claims | Claim	34,349
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 291	262	223	169	$ 71
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(17)	(1)	(12)	(3)
IBNR, Total	(33)
Workers’ compensation | Short-Duration Insurance Contract, Accident Year 2020
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	414	446	466	477
IBNR	$ 135
Cumulative number of claims | Claim	29,454
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 228	200	147	$ 65
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(32)	(20)	(11)
IBNR, Total	(63)
Workers’ compensation | Short-Duration Insurance Contract, Accident Year 2021
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	432	454	468
IBNR	$ 146
Cumulative number of claims | Claim	30,066
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 222	164	$ 67
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(22)	(14)
IBNR, Total	(36)
Workers’ compensation | Short-Duration Insurance Contract, Accident Year 2022
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	489	497
IBNR	$ 198
Cumulative number of claims | Claim	33,229
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 192	$ 79
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(8)
IBNR, Total	(8)
Workers’ compensation | Short-Duration Insurance Contract, Accident Year 2023
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	555
IBNR	$ 344
Cumulative number of claims | Claim	31,549
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 87