Claim, Claim Adjustment Expense and Future Policy Benefit Reserves, Surety (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
Surety [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	766
IBNR	290
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	415
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	351
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2011	14
Liability for unallocated claim adjustment expenses for accident years presented	20
Total net liability for unpaid claim and claim adjustment expenses	385
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total net development for the accident years presented above	(67)	(79)	[1]	(68)	[1]
Total net development for accident years prior to 2011	(2)	(3)	[1]	(2)	[1]
Total unallocated claim adjustment expense development	0	(10)	[1]	0	[1]
Total	(69)	(92)	[1]	(70)	[1]
Surety [Member] | Accident Year 2011 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	62	62	[1]	62	[1]	66	[1]	$ 70		$ 75		$ 87		$ 116		$ 121		$ 120
IBNR	$ 1
Cumulative number of claims | Claim	5,828
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 57	57	[1]	57	[1]	56	[1]	60		60		58		55		42		$ 19
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	0	0	[1]	(4)	[1]	(4)	[1]	(5)		(12)		(29)		(5)		1
IBNR, Total	(58)
Surety [Member] | Accident Year 2012 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	37	38	[1]	39	[1]	45	[1]	52		70		98		122		120
IBNR	$ 1
Cumulative number of claims | Claim	5,577
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 36	37	[1]	37	[1]	36	[1]	35		35		34		32		$ 5
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(1)	(1)	[1]	(6)	[1]	(7)	[1]	(18)		(28)		(24)		2
IBNR, Total	(83)
Surety [Member] | Accident Year 2013 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	82	83	[1]	87	[1]	91	[1]	106		115		121		120
IBNR	$ 1
Cumulative number of claims | Claim	5,078
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 78	77	[1]	78	[1]	78	[1]	78		69		40		$ 16
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(1)	(4)	[1]	(4)	[1]	(15)	[1]	(9)		(6)		1
IBNR, Total	(38)
Surety [Member] | Accident Year 2014 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	45	45	[1]	60	[1]	69	[1]	94		124		123
IBNR	$ 1
Cumulative number of claims | Claim	5,102
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 39	38	[1]	38	[1]	36	[1]	38		30		$ 7
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	0	(15)	[1]	(9)	[1]	(25)	[1]	(30)		1
IBNR, Total	(78)
Surety [Member] | Accident Year 2015 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	58	63	[1]	79	[1]	104	[1]	131		131
IBNR	$ 6
Cumulative number of claims | Claim	5,026
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 44	42	[1]	40	[1]	38	[1]	26		$ 7
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(5)	(16)	[1]	(25)	[1]	(27)	[1]	0
IBNR, Total	(73)
Surety [Member] | Accident Year 2016 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	67	84	[1]	109	[1]	124	[1]	124
IBNR	$ 12
Cumulative number of claims | Claim	5,469
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 43	45	[1]	45	[1]	37	[1]	$ 5
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(17)	(25)	[1]	(15)	[1]	0	[1]
IBNR, Total	(57)
Surety [Member] | Accident Year 2017 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	84	103	[1]	115	[1]	120	[1]
IBNR	$ 31
Cumulative number of claims | Claim	5,706
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 46	41	[1]	37	[1]	$ 23	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(19)	(12)	[1]	(5)	[1]
IBNR, Total	(36)
Surety [Member] | Accident Year 2018 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	91	108	[1]	114	[1]
IBNR	$ 48
Cumulative number of claims | Claim	5,920
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 34	25	[1]	$ 5	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(17)	(6)	[1]
IBNR, Total	(23)
Surety [Member] | Accident Year 2019 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	112	119	[1]
IBNR	$ 67
Cumulative number of claims | Claim	5,344
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 34	$ 12	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(7)
IBNR, Total	(7)
Surety [Member] | Accident Year 2020 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	128
IBNR	$ 122
Cumulative number of claims | Claim	2,527
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 4

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