Claim, Claim Adjustment Expense and Future Policy Benefit Reserves, Commercial Auto (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
Commercial Auto [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	2,425
IBNR	262
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	1,939
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	486
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2011	2
Liability for unallocated claim adjustment expenses for accident years presented	14
Total net liability for unpaid claim and claim adjustment expenses	502
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total net development for the accident years presented above	32	(20)	[1]	(1)	[1]
Total net development for accident years prior to 2011	1	(4)	[1]	1	[1]
Total unallocated claim adjustment expense development	0	(1)	[1]	1	[1]
Total	33	(25)	[1]	1	[1]
Commercial Auto [Member] | Accident Year 2011 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	292	291	[1]	294	[1]	294	[1]	$ 294		$ 300		$ 302		$ 288		$ 281		$ 268
IBNR	$ 0
Cumulative number of claims | Claim	47,907
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 290	289	[1]	289	[1]	287	[1]	284		274		248		199		145		$ 79
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	1	(3)	[1]	0	[1]	0	[1]	(6)		(2)		14		7		13
Total	24
Commercial Auto [Member] | Accident Year 2012 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	296	297	[1]	299	[1]	299	[1]	307		303		299		289		275
IBNR	$ 2
Cumulative number of claims | Claim	46,288
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 291	291	[1]	290	[1]	285	[1]	282		259		220		160		$ 78
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(1)	(2)	[1]	0	[1]	(8)	[1]	4		4		10		14
Total	21
Commercial Auto [Member] | Accident Year 2013 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	241	241	[1]	245	[1]	245	[1]	249		265		265		246
IBNR	$ 2
Cumulative number of claims | Claim	39,429
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 239	238	[1]	234	[1]	225	[1]	200		168		135		$ 74
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	0	(4)	[1]	0	[1]	(4)	[1]	(16)		0		19
Total	(5)
Commercial Auto [Member] | Accident Year 2014 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	201	201	[1]	205	[1]	205	[1]	212		223		234
IBNR	$ 1
Cumulative number of claims | Claim	33,625
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 198	196	[1]	187	[1]	166	[1]	137		102		$ 64
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	0	(4)	[1]	0	[1]	(7)	[1]	(11)		(11)
Total	(33)
Commercial Auto [Member] | Accident Year 2015 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	181	183	[1]	190	[1]	190	[1]	199		201
IBNR	$ 3
Cumulative number of claims | Claim	30,426
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 175	172	[1]	153	[1]	130	[1]	96		$ 52
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(2)	(7)	[1]	0	[1]	(9)	[1]	(2)
Total	(20)
Commercial Auto [Member] | Accident Year 2016 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	190	186	[1]	186	[1]	186	[1]	198
IBNR	$ 2
Cumulative number of claims | Claim	30,430
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 175	154	[1]	126	[1]	93	[1]	$ 52
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	4	0	[1]	0	[1]	(12)	[1]
Total	(8)
Commercial Auto [Member] | Accident Year 2017 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	221	200	[1]	198	[1]	199	[1]
IBNR	$ 7
Cumulative number of claims | Claim	30,913
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 178	150	[1]	107	[1]	$ 58	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	21	2	[1]	(1)	[1]
Total	22
Commercial Auto [Member] | Accident Year 2018 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	227	227	[1]	229	[1]
IBNR	$ 10
Cumulative number of claims | Claim	34,225
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 175	128	[1]	$ 66	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	0	(2)	[1]
Total	(2)
Commercial Auto [Member] | Accident Year 2019 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	266	257	[1]
IBNR	$ 48
Cumulative number of claims | Claim	36,779
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 147	$ 77	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	9
Total	9
Commercial Auto [Member] | Accident Year 2020 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	310
IBNR	$ 187
Cumulative number of claims | Claim	24,427
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 71

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