Claim, Claim Adjustment Expense and Future Policy Benefit Reserves, Medical Professional 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
Medical Professional Liability [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	4,825
IBNR	905
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	3,353
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,472
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2011	19
Liability for unallocated claim adjustment expenses for accident years presented	29
Total net liability for unpaid claim and claim adjustment expenses	1,520
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total net development for the accident years presented above	32	54	[1]	43	[1]
Total net development for accident years prior to 2011	3	19	[1]	5	[1]
Total unallocated claim adjustment expense development	0	2	[1]	(1)	[1]
Total	35	75	[1]	47	[1]
Medical Professional Liability [Member] | Accident Year 2011 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	439	439	[1]	437	[1]	437	[1]	$ 434		$ 439		$ 468		$ 443		$ 437		$ 429
IBNR	$ 4
Cumulative number of claims | Claim	16,537
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 432	414	[1]	409	[1]	398	[1]	375		347		295		208		109		$ 17
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	0	2	[1]	0	[1]	3	[1]	(5)		(29)		25		6		8
Total	10
Medical Professional Liability [Member] | Accident Year 2012 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	497	499	[1]	493	[1]	484	[1]	493		498		508		469		464
IBNR	$ 5
Cumulative number of claims | Claim	17,739
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 482	479	[1]	457	[1]	427	[1]	388		323		221		117		$ 14
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(2)	6	[1]	9	[1]	(9)	[1]	(5)		(10)		39		5
Total	33
Medical Professional Liability [Member] | Accident Year 2013 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	531	545	[1]	535	[1]	525	[1]	513		500		479		462
IBNR	$ 11
Cumulative number of claims | Claim	19,537
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 508	495	[1]	462	[1]	414	[1]	355		255		119		$ 17
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(14)	10	[1]	10	[1]	12	[1]	13		21		17
Total	69
Medical Professional Liability [Member] | Accident Year 2014 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	527	529	[1]	535	[1]	530	[1]	537		489		450
IBNR	$ 11
Cumulative number of claims | Claim	19,770
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 489	472	[1]	417	[1]	359	[1]	258		136		$ 23
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(2)	(6)	[1]	5	[1]	(7)	[1]	48		39
Total	77
Medical Professional Liability [Member] | Accident Year 2015 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	510	488	[1]	494	[1]	510	[1]	499		433
IBNR	$ 29
Cumulative number of claims | Claim	18,122
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 420	384	[1]	313	[1]	230	[1]	101		$ 22
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	22	(6)	[1]	(16)	[1]	11	[1]	66
Total	77
Medical Professional Liability [Member] | Accident Year 2016 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	508	499	[1]	485	[1]	487	[1]	427
IBNR	$ 27
Cumulative number of claims | Claim	15,998
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 401	339	[1]	246	[1]	121	[1]	$ 18
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	9	14	[1]	(2)	[1]	60	[1]
Total	81
Medical Professional Liability [Member] | Accident Year 2017 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	460	458	[1]	449	[1]	412	[1]
IBNR	$ 62
Cumulative number of claims | Claim	15,008
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 308	235	[1]	107	[1]	$ 19	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	2	9	[1]	37	[1]
Total	48
Medical Professional Liability [Member] | Accident Year 2018 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	431	429	[1]	404	[1]
IBNR	$ 98
Cumulative number of claims | Claim	14,531
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 211	115	[1]	$ 21	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	2	25	[1]
Total	27
Medical Professional Liability [Member] | Accident Year 2019 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	445	430	[1]
IBNR	$ 232
Cumulative number of claims | Claim	13,045
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 91	$ 17	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	15
Total	15
Medical Professional Liability [Member] | Accident Year 2020 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	477
IBNR	$ 426
Cumulative number of claims | Claim	7,787
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 11

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