Claim, Claim Adjustment Expense and Future Policy Benefit Reserves, Workers' Compensation (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 ($)	[2]	Dec. 31, 2015 USD ($)	[2]	Dec. 31, 2014 USD ($)	[2]	Dec. 31, 2013 USD ($)	[2]	Dec. 31, 2012 USD ($)	[2]	Dec. 31, 2011 USD ($)	[2]
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
Workers' Compensation [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		4,854
IBNR		1,249
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		3,000
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented		1,854
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2011		1,984
Other	[1]	(15)
Liability for unallocated claim adjustment expenses for accident years presented		49
Total net liability for unpaid claim and claim adjustment expenses		3,872
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total net development for the accident years presented above		(75)	(63)	[2]	(32)	[2]
Adjustment for development on a discounted basis		2	3	[2]	0	[2]
Total net development for accident years prior to 2011		(23)	24	[2]	7	[2]
Total unallocated claim adjustment expense development		0	23	[2]	(7)	[2]
Total		(96)	(13)	[2]	(32)	[2]
Workers' Compensation [Member] | Accident Year 2011 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		698	688	[2]	674	[2]	676	[2]	$ 676		$ 651		$ 659		$ 647		$ 641		$ 607
IBNR		$ 42
Cumulative number of claims | Claim		46,443
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		$ 583	581	[2]	571	[2]	564	[2]	522		478		438		358		249		$ 99
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total		10	14	[2]	(2)	[2]	0	[2]	25		(8)		12		6		34
Total		91
Workers' Compensation [Member] | Accident Year 2012 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		663	668	[2]	671	[2]	673	[2]	678		669		659		627		601
IBNR		$ 62
Cumulative number of claims | Claim		42,685
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		$ 538	536	[2]	524	[2]	509	[2]	470		416		342		232		$ 87
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total		(5)	(3)	[2]	(2)	[2]	(5)	[2]	9		10		32		26
Total		62
Workers' Compensation [Member] | Accident Year 2013 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		552	561	[2]	582	[2]	593	[2]	618		592		572		537
IBNR		$ 92
Cumulative number of claims | Claim		38,758
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		$ 414	411	[2]	419	[2]	417	[2]	370		300		213		$ 80
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total		(9)	(21)	[2]	(11)	[2]	(25)	[2]	26		20		35
Total		15
Workers' Compensation [Member] | Accident Year 2014 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		439	446	[2]	450	[2]	452	[2]	479		480		467
IBNR		$ 97
Cumulative number of claims | Claim		33,488
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		$ 297	290	[2]	282	[2]	258	[2]	215		159		$ 61
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total		(7)	(4)	[2]	(2)	[2]	(27)	[2]	(1)		13
Total		(28)
Workers' Compensation [Member] | Accident Year 2015 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		382	394	[2]	408	[2]	406	[2]	431		422
IBNR		$ 116
Cumulative number of claims | Claim		31,876
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		$ 243	231	[2]	212	[2]	180	[2]	131		$ 51
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total		(12)	(14)	[2]	2	[2]	(25)	[2]	9
Total		(40)
Workers' Compensation [Member] | Accident Year 2016 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		366	382	[2]	396	[2]	405	[2]	426
IBNR		$ 121
Cumulative number of claims | Claim		31,967
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		$ 219	198	[2]	169	[2]	129	[2]	$ 53
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total		(16)	(14)	[2]	(9)	[2]	(21)	[2]
Total		(60)
Workers' Compensation [Member] | Accident Year 2017 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		400	421	[2]	432	[2]	440	[2]
IBNR		$ 97
Cumulative number of claims | Claim		33,094
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		$ 243	207	[2]	151	[2]	$ 63	[2]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total		(21)	(11)	[2]	(8)	[2]
Total		(40)
Workers' Compensation [Member] | Accident Year 2018 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		428	440	[2]	450	[2]
IBNR		$ 129
Cumulative number of claims | Claim		34,800
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		$ 229	163	[2]	$ 68	[2]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total		(12)	(10)	[2]
Total		(22)
Workers' Compensation [Member] | Accident Year 2019 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		449	452	[2]
IBNR		$ 181
Cumulative number of claims | Claim		34,020
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		$ 169	$ 71	[2]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total		(3)
Total		(3)
Workers' Compensation [Member] | Accident Year 2020 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		477
IBNR		$ 312
Cumulative number of claims | Claim		24,980
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total		$ 65

[1] Other includes the effect of discounting lifetime claim reserves.

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