Claim and Claim Adjustment Expense Reserves and Future Policy Benefit Reserves - Other Professional Liability and Management Liability (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
Other Professional Liability and Management Liability
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	9,080
IBNR	2,673
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	5,721
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	3,359
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2013	99
Liability for unallocated claim adjustment expenses for accident years presented	56
Total net liability for unpaid claim and claim adjustment expenses	3,514
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total net development for the accident years presented above	41	36	(13)
Total net development for accident years prior to 2013	9	(14)	(2)
Total unallocated claim adjustment expense development	0	2	0
Total	50	24	(15)
Other Professional Liability and Management Liability | Short-duration Insurance Contracts, Accident Year 2013
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	829	833	846	850	$ 863	$ 866	$ 885	$ 926	$ 894	$ 884
IBNR	$ 21
Cumulative number of claims | Claim	17,953
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 791	787	779	771	754	702	618	447	249	$ 54
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(4)	(13)	(4)	(13)	(3)	(19)	(41)	32	10
IBNR, Total	(55)
Other Professional Liability and Management Liability | Short-duration Insurance Contracts, Accident Year 2014
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	842	841	845	854	835	831	885	898	878
IBNR	$ 29
Cumulative number of claims | Claim	17,583
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 802	787	743	707	647	515	392	223	$ 51
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	1	(4)	(9)	19	4	(54)	(13)	20
IBNR, Total	(36)
Other Professional Liability and Management Liability | Short-duration Insurance Contracts, Accident Year 2015
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	855	836	813	807	832	877	892	888
IBNR	$ 30
Cumulative number of claims | Claim	17,452
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 794	725	677	612	542	404	234	$ 60
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	19	23	6	(25)	(45)	(15)	4
IBNR, Total	(33)
Other Professional Liability and Management Liability | Short-duration Insurance Contracts, Accident Year 2016
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	888	891	907	904	900	900	901
IBNR	$ 57
Cumulative number of claims | Claim	17,976
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 784	736	701	625	466	248	$ 64
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(3)	(16)	3	4	0	(1)
IBNR, Total	(13)
Other Professional Liability and Management Liability | Short-duration Insurance Contracts, Accident Year 2017
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	758	775	791	813	845	847
IBNR	$ 107
Cumulative number of claims | Claim	18,181
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 596	557	498	394	222	$ 57
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(17)	(16)	(22)	(32)	(2)
IBNR, Total	(89)
Other Professional Liability and Management Liability | Short-duration Insurance Contracts, Accident Year 2018
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	923	906	869	864	850
IBNR	$ 142
Cumulative number of claims | Claim	19,995
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 706	599	473	282	$ 54
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	17	37	5	14
IBNR, Total	73
Other Professional Liability and Management Liability | Short-Duration Insurance Contract, Accident Year 2019
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	876	856	845	837
IBNR	$ 173
Cumulative number of claims | Claim	19,447
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 567	422	263	$ 64
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	20	11	8
IBNR, Total	39
Other Professional Liability and Management Liability | Short-Duration Insurance Contract, Accident Year 2020
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	951	944	930
IBNR	$ 408
Cumulative number of claims | Claim	19,333
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 400	248	$ 67
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	7	14
IBNR, Total	21
Other Professional Liability and Management Liability | Short-Duration Insurance Contract, Accident Year 2021
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	1,038	1,037
IBNR	$ 706
Cumulative number of claims | Claim	17,983
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 217	$ 58
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	1
IBNR, Total	1
Other Professional Liability and Management Liability | Short-Duration Insurance Contract, Accident Year 2022
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	1,120
IBNR	$ 1,000
Cumulative number of claims | Claim	15,327
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 64