Claim, Claim Adjustment Expense and Future Policy Benefit Reserves, General 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
General Liability [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	6,541
IBNR	1,789
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	4,108
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	2,433
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2011	812
Liability for unallocated claim adjustment expenses for accident years presented	60
Total net liability for unpaid claim and claim adjustment expenses	3,305
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total net development for the accident years presented above	16	19	[1]	36	[1]
Total net development for accident years prior to 2011	49	28	[1]	0	[1]
Total unallocated claim adjustment expense development	0	7	[1]	(4)	[1]
Total	65	54	[1]	32	[1]
General Liability [Member] | Accident Year 2011 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	667	667	[1]	669	[1]	670	[1]	$ 681		$ 676		$ 677		$ 631		$ 589		$ 591
IBNR	$ 21
Cumulative number of claims | Claim	39,405
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 640	638	[1]	622	[1]	602	[1]	568		517		411		273		148		$ 28
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	0	(2)	[1]	(1)	[1]	(11)	[1]	5		(1)		46		42		(2)
Total	76
General Liability [Member] | Accident Year 2012 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	632	630	[1]	635	[1]	635	[1]	619		636		639		611		587
IBNR	$ 24
Cumulative number of claims | Claim	35,276
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 597	579	[1]	559	[1]	510	[1]	454		374		247		132		$ 28
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	2	(5)	[1]	0	[1]	16	[1]	(17)		(3)		28		24
Total	45
General Liability [Member] | Accident Year 2013 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	623	620	[1]	623	[1]	613	[1]	655		650		655		650
IBNR	$ 27
Cumulative number of claims | Claim	33,649
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 572	551	[1]	510	[1]	450	[1]	352		240		128		$ 31
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	3	(3)	[1]	10	[1]	(42)	[1]	5		(5)		5
Total	(27)
General Liability [Member] | Accident Year 2014 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	659	658	[1]	635	[1]	631	[1]	654		658		653
IBNR	$ 44
Cumulative number of claims | Claim	27,972
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 569	547	[1]	481	[1]	376	[1]	247		119		$ 31
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	1	23	[1]	4	[1]	(23)	[1]	(4)		5
Total	6
General Liability [Member] | Accident Year 2015 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	602	600	[1]	589	[1]	574	[1]	576		581
IBNR	$ 38
Cumulative number of claims | Claim	24,005
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 501	446	[1]	357	[1]	230	[1]	110		$ 19
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	2	11	[1]	15	[1]	(2)	[1]	(5)
Total	21
General Liability [Member] | Accident Year 2016 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	673	671	[1]	667	[1]	659	[1]	623
IBNR	$ 104
Cumulative number of claims | Claim	24,215
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 481	407	[1]	279	[1]	163	[1]	$ 32
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	2	4	[1]	8	[1]	36	[1]
Total	50
General Liability [Member] | Accident Year 2017 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	634	632	[1]	632	[1]	632	[1]
IBNR	$ 136
Cumulative number of claims | Claim	21,781
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 399	250	[1]	118	[1]	$ 23	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	2	0	[1]	0	[1]
Total	2
General Liability [Member] | Accident Year 2018 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	646	644	[1]	653	[1]
IBNR	$ 302
Cumulative number of claims | Claim	19,234
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 228	107	[1]	$ 33	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	2	(9)	[1]
Total	(7)
General Liability [Member] | Accident Year 2019 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	682	680	[1]
IBNR	$ 453
Cumulative number of claims | Claim	17,294
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 98	$ 25	[1]
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	2
Total	2
General Liability [Member] | Accident Year 2020 [Member]
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	723
IBNR	$ 640
Cumulative number of claims | Claim	9,593
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 23

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