Claim, Claim Adjustment Expense and Future Policy Benefit Reserves, Medical Professional 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
Medical Professional Liability
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	4,776
IBNR	1,016
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	3,276
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,500
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2012	28
Liability for unallocated claim adjustment expenses for accident years presented	28
Total net liability for unpaid claim and claim adjustment expenses	1,556
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total net development for the accident years presented above	13	32	52
Total net development for accident years prior to 2012	3	3	21
Total unallocated claim adjustment expense development	7	0	2
Total	23	35	75
Medical Professional Liability | Short-duration Insurance Contracts, Accident Year 2012
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	497	497	499	493	$ 484	$ 493	$ 498	$ 508	$ 469	$ 464
IBNR	$ 4
Cumulative number of claims | Claim	17,755
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 487	482	479	457	427	388	323	221	117	$ 14
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	0	(2)	6	9	(9)	(5)	(10)	39	5
IBNR, Total	33
Medical Professional Liability | Short-duration Insurance Contracts, Accident Year 2013
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	530	531	545	535	525	513	500	479	462
IBNR	$ 11
Cumulative number of claims | Claim	19,565
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 512	508	495	462	414	355	255	119	$ 17
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(1)	(14)	10	10	12	13	21	17
IBNR, Total	68
Medical Professional Liability | Short-duration Insurance Contracts, Accident Year 2014
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	524	527	529	535	530	537	489	450
IBNR	$ 8
Cumulative number of claims | Claim	19,800
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 497	489	472	417	359	258	136	$ 23
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(3)	(2)	(6)	5	(7)	48	39
IBNR, Total	74
Medical Professional Liability | Short-duration Insurance Contracts, Accident Year 2015
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	501	510	488	494	510	499	433
IBNR	$ 28
Cumulative number of claims | Claim	18,170
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 444	420	384	313	230	101	$ 22
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(9)	22	(6)	(16)	11	66
IBNR, Total	68
Medical Professional Liability | Short-duration Insurance Contracts, Accident Year 2016
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	510	508	499	485	487	427
IBNR	$ 24
Cumulative number of claims | Claim	16,085
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 436	401	339	246	121	$ 18
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	2	9	14	(2)	60
IBNR, Total	83
Medical Professional Liability | Short-duration Insurance Contracts, Accident Year 2017
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	455	460	458	449	412
IBNR	$ 41
Cumulative number of claims | Claim	15,197
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 355	308	235	107	$ 19
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(5)	2	9	37
IBNR, Total	43
Medical Professional Liability | Short-duration Insurance Contracts, Accident Year 2018
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	448	431	429	404
IBNR	$ 60
Cumulative number of claims | Claim	14,997
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 290	211	115	$ 21
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	17	2	25
IBNR, Total	44
Medical Professional Liability | Short-Duration Insurance Contract, Accident Year 2019
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	458	445	430
IBNR	$ 156
Cumulative number of claims | Claim	13,804
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 183	91	$ 17
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	13	15
IBNR, Total	28
Medical Professional Liability | Short-Duration Insurance Contract, Accident Year 2020
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	476	477
IBNR	$ 347
Cumulative number of claims | Claim	9,935
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 61	$ 11
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(1)
IBNR, Total	(1)
Medical Professional Liability | Short-Duration Insurance Contract, Accident Year 2021
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	377
IBNR	$ 337
Cumulative number of claims | Claim	6,761
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 11