Claim, Claim Adjustment Expense and Future Policy Benefit Reserves, Other Professional Liability and Management Liability (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) Claim	Dec. 31, 2019 USD ($)		Dec. 31, 2018 USD ($)		Dec. 31, 2017 USD ($)		Dec. 31, 2016 USD ($)	[1]	Dec. 31, 2015 USD ($)	[1]	Dec. 31, 2014 USD ($)	[1]	Dec. 31, 2013 USD ($)	[1]	Dec. 31, 2012 USD ($)	[1]	Dec. 31, 2011 USD ($)	[1]
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2011	$ 4,216	$ 4,584		$ 4,285
Total net liability for unpaid claim and claim adjustment expenses	18,701	17,885		17,965		$ 18,070
Property and Casualty Operations [Member]
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total net liability for unpaid claim and claim adjustment expenses	14,924	14,153
Other Professional Liability and Management Liability [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	8,579
IBNR	2,049
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	5,864
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	2,715
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2011	79
Liability for unallocated claim adjustment expenses for accident years presented	56
Total net liability for unpaid claim and claim adjustment expenses	2,850
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total net development for the accident years presented above	4	(38)	[1]	(70)	[1]
Total net development for accident years prior to 2011	(19)	(17)	[1]	(50)	[1]
Total unallocated claim adjustment expense development	0	(14)	[1]	(7)	[1]
Total	(15)	(69)	[1]	(127)	[1]
Other Professional Liability and Management Liability [Member] | Accident Year 2011 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	883	885	[1]	888	[1]	899	[1]	$ 911		$ 944		$ 949		$ 934		$ 908		$ 880
IBNR	$ 17
Cumulative number of claims | Claim	18,745
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 851	828	[1]	796	[1]	781	[1]	726		683		605		503		314		$ 71
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(2)	(3)	[1]	(11)	[1]	(12)	[1]	(33)		(5)		15		26		28
IBNR, Total	3
Other Professional Liability and Management Liability [Member] | Accident Year 2012 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	850	831	[1]	833	[1]	846	[1]	840		878		887		909		923
IBNR	$ 25
Cumulative number of claims | Claim	18,504
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 812	792	[1]	755	[1]	711	[1]	651		573		400		248		$ 56
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	19	(2)	[1]	(13)	[1]	6	[1]	(38)		(9)		(22)		(14)
IBNR, Total	(73)
Other Professional Liability and Management Liability [Member] | Accident Year 2013 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	846	850	[1]	863	[1]	866	[1]	885		926		894		884
IBNR	$ 36
Cumulative number of claims | Claim	17,939
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 779	771	[1]	754	[1]	702	[1]	618		447		249		$ 54
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(4)	(13)	[1]	(3)	[1]	(19)	[1]	(41)		32		10
IBNR, Total	(38)
Other Professional Liability and Management Liability [Member] | Accident Year 2014 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	845	854	[1]	835	[1]	831	[1]	885		898		878
IBNR	$ 57
Cumulative number of claims | Claim	17,568
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 743	707	[1]	647	[1]	515	[1]	392		223		$ 51
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(9)	19	[1]	4	[1]	(54)	[1]	(13)		20
IBNR, Total	(33)
Other Professional Liability and Management Liability [Member] | Accident Year 2015 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	813	807	[1]	832	[1]	877	[1]	892		888
IBNR	$ 74
Cumulative number of claims | Claim	17,417
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 677	612	[1]	542	[1]	404	[1]	234		$ 60
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	6	(25)	[1]	(45)	[1]	(15)	[1]	4
IBNR, Total	(75)
Other Professional Liability and Management Liability [Member] | Accident Year 2016 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	907	904	[1]	900	[1]	900	[1]	901
IBNR	$ 120
Cumulative number of claims | Claim	17,946
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 701	625	[1]	466	[1]	248	[1]	$ 64
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	3	4	[1]	0	[1]	(1)	[1]
IBNR, Total	6
Other Professional Liability and Management Liability [Member] | Accident Year 2017 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	791	813	[1]	845	[1]	847	[1]
IBNR	$ 220
Cumulative number of claims | Claim	18,118
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 498	394	[1]	222	[1]	$ 57	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(22)	(32)	[1]	(2)	[1]
IBNR, Total	(56)
Other Professional Liability and Management Liability [Member] | Accident Year 2018 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	869	864	[1]	850	[1]
IBNR	$ 276
Cumulative number of claims | Claim	19,789
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 473	282	[1]	$ 54	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	5	14	[1]
IBNR, Total	19
Other Professional Liability and Management Liability [Member] | Accident Year 2019 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	845	837	[1]
IBNR	$ 447
Cumulative number of claims | Claim	19,157
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 263	$ 64	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	8
IBNR, Total	8
Other Professional Liability and Management Liability [Member] | Accident Year 2020 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	930
IBNR	$ 777
Cumulative number of claims | Claim	16,557
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 67

[1] Data presented for these calendar years is required supplemental information, which is unaudited.