Claim and Claim Adjustment Expense Reserves and Future Policy Benefit Reserves - Workers' Compensation (Details) $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($) Claim	Dec. 31, 2021 USD ($)	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 ($)	Dec. 31, 2013 USD ($)
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2013	$ 4,348	$ 3,830	$ 4,216
Total net liability for unpaid claim and claim adjustment expenses	19,908	19,205	18,701	$ 17,885
Property and casualty
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total net liability for unpaid claim and claim adjustment expenses	15,996	15,270
Workers' Compensation
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	4,298
IBNR	1,286
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	2,489
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,809
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2013	1,874
Other	(20)
Liability for unallocated claim adjustment expenses for accident years presented	76
Total net liability for unpaid claim and claim adjustment expenses	3,739
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total net development for the accident years presented above	(110)	(50)	(80)
Adjustment for development on a discounted basis	(3)	2	2
Total net development for accident years prior to 2013	(49)	(34)	(18)
Total unallocated claim adjustment expense development	10	0	0
Total	(152)	(82)	(96)
Workers' Compensation | Short-duration Insurance Contracts, Accident Year 2013
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	537	548	552	561	$ 582	$ 593	$ 618	$ 592	$ 572	$ 537
IBNR	$ 82
Cumulative number of claims | Claim	38,979
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 423	417	414	411	419	417	370	300	213	$ 80
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(11)	(4)	(9)	(21)	(11)	(25)	26	20	35
IBNR, Total	0
Workers' Compensation | Short-duration Insurance Contracts, Accident Year 2014
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	430	448	439	446	450	452	479	480	467
IBNR	$ 76
Cumulative number of claims | Claim	33,522
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 312	306	297	290	282	258	215	159	$ 61
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(18)	9	(7)	(4)	(2)	(27)	(1)	13
IBNR, Total	(37)
Workers' Compensation | Short-duration Insurance Contracts, Accident Year 2015
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	353	372	382	394	408	406	431	422
IBNR	$ 80
Cumulative number of claims | Claim	31,899
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 256	251	243	231	212	180	131	$ 51
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(19)	(10)	(12)	(14)	2	(25)	9
IBNR, Total	(69)
Workers' Compensation | Short-duration Insurance Contracts, Accident Year 2016
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	331	355	366	382	396	405	426
IBNR	$ 77
Cumulative number of claims | Claim	31,991
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 234	227	219	198	169	129	$ 53
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(24)	(11)	(16)	(14)	(9)	(21)
IBNR, Total	(95)
Workers' Compensation | Short-duration Insurance Contracts, Accident Year 2017
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	399	402	400	421	432	440
IBNR	$ 81
Cumulative number of claims | Claim	33,130
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 279	265	243	207	151	$ 63
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(3)	2	(21)	(11)	(8)
IBNR, Total	(41)
Workers' Compensation | Short-duration Insurance Contracts, Accident Year 2018
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	415	415	428	440	450
IBNR	$ 93
Cumulative number of claims | Claim	34,875
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 280	259	229	163	$ 68
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	0	(13)	(12)	(10)
IBNR, Total	(35)
Workers' Compensation | Short-Duration Insurance Contract, Accident Year 2019
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	436	437	449	452
IBNR	$ 105
Cumulative number of claims | Claim	34,324
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 262	223	169	$ 71
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(1)	(12)	(3)
IBNR, Total	(16)
Workers' Compensation | Short-Duration Insurance Contract, Accident Year 2020
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	446	466	477
IBNR	$ 182
Cumulative number of claims | Claim	29,392
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 200	147	$ 65
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(20)	(11)
IBNR, Total	(31)
Workers' Compensation | Short-Duration Insurance Contract, Accident Year 2021
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	454	468
IBNR	$ 202
Cumulative number of claims | Claim	29,886
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 164	$ 67
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(14)
IBNR, Total	(14)
Workers' Compensation | Short-Duration Insurance Contract, Accident Year 2022
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	497
IBNR	$ 308
Cumulative number of claims | Claim	28,753
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 79