Claim, Claim Adjustment Expense and Future Policy Benefit Reserves, Other Professional Liability and Management Liability (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) Claim	Dec. 31, 2020 USD ($)	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 ($)	Dec. 31, 2012 USD ($)
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2012	$ 3,830	$ 4,216	$ 4,584
Total net liability for unpaid claim and claim adjustment expenses	19,205	18,701	17,885	$ 17,965
Property and casualty
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total net liability for unpaid claim and claim adjustment expenses	15,270	14,195
Other Professional Liability and Management Liability
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	8,767
IBNR	2,326
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	5,735
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	3,032
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2012	74
Liability for unallocated claim adjustment expenses for accident years presented	53
Total net liability for unpaid claim and claim adjustment expenses	3,159
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total net development for the accident years presented above	34	6	(35)
Total net development for accident years prior to 2012	(12)	(21)	(20)
Total unallocated claim adjustment expense development	2	0	(14)
Total	24	(15)	(69)
Other Professional Liability and Management Liability | Short-duration Insurance Contracts, Accident Year 2012
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	848	850	831	833	$ 846	$ 840	$ 878	$ 887	$ 909	$ 923
IBNR	$ 18
Cumulative number of claims | Claim	18,506
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 816	812	792	755	711	651	573	400	248	$ 56
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(2)	19	(2)	(13)	6	(38)	(9)	(22)	(14)
IBNR, Total	(75)
Other Professional Liability and Management Liability | Short-duration Insurance Contracts, Accident Year 2013
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	833	846	850	863	866	885	926	894	884
IBNR	$ 30
Cumulative number of claims | Claim	17,950
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 787	779	771	754	702	618	447	249	$ 54
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(13)	(4)	(13)	(3)	(19)	(41)	32	10
IBNR, Total	(51)
Other Professional Liability and Management Liability | Short-duration Insurance Contracts, Accident Year 2014
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	841	845	854	835	831	885	898	878
IBNR	$ 37
Cumulative number of claims | Claim	17,577
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 787	743	707	647	515	392	223	$ 51
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(4)	(9)	19	4	(54)	(13)	20
IBNR, Total	(37)
Other Professional Liability and Management Liability | Short-duration Insurance Contracts, Accident Year 2015
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	836	813	807	832	877	892	888
IBNR	$ 41
Cumulative number of claims | Claim	17,436
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 725	677	612	542	404	234	$ 60
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	23	6	(25)	(45)	(15)	4
IBNR, Total	(52)
Other Professional Liability and Management Liability | Short-duration Insurance Contracts, Accident Year 2016
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	891	907	904	900	900	901
IBNR	$ 84
Cumulative number of claims | Claim	17,968
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 736	701	625	466	248	$ 64
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(16)	3	4	0	(1)
IBNR, Total	(10)
Other Professional Liability and Management Liability | Short-duration Insurance Contracts, Accident Year 2017
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	775	791	813	845	847
IBNR	$ 152
Cumulative number of claims | Claim	18,159
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 557	498	394	222	$ 57
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(16)	(22)	(32)	(2)
IBNR, Total	(72)
Other Professional Liability and Management Liability | Short-duration Insurance Contracts, Accident Year 2018
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	906	869	864	850
IBNR	$ 202
Cumulative number of claims | Claim	19,926
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 599	473	282	$ 54
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	37	5	14
IBNR, Total	56
Other Professional Liability and Management Liability | Short-Duration Insurance Contract, Accident Year 2019
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	856	845	837
IBNR	$ 283
Cumulative number of claims | Claim	19,357
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 422	263	$ 64
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	11	8
IBNR, Total	19
Other Professional Liability and Management Liability | Short-Duration Insurance Contract, Accident Year 2020
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	944	930
IBNR	$ 568
Cumulative number of claims | Claim	19,095
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 248	$ 67
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	14
IBNR, Total	14
Other Professional Liability and Management Liability | Short-Duration Insurance Contract, Accident Year 2021
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	1,037
IBNR	$ 911
Cumulative number of claims | Claim	15,487
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 58