Basis of Presentation (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2020		Jun. 30, 2019	Jun. 30, 2020		Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Basis of Presentation [Abstract]
Cumulative effect of accounting change	$ 18,413		$ 22,394	$ 18,413		$ 22,394	$ 19,178	$ 21,930	$ 21,902	$ 21,386
Allowance for credit losses	51	[1]		$ 51	[1]		49	0
Period of time after interest becomes delinquent that asset is placed on nonaccrual status				90 days
ASU 2016-13 [Member]
Basis of Presentation [Abstract]
Allowance for credit losses								6
Retained Earnings [Member]
Basis of Presentation [Abstract]
Cumulative effect of accounting change	$ 14,316		$ 16,374	$ 14,316		$ 16,374	$ 15,167	15,823	$ 16,144	$ 15,773
Cumulative Effect, Period of Adoption, Adjustment [Member] | ASU 2016-13 [Member]
Basis of Presentation [Abstract]
Cumulative effect of accounting change								(5)
Allowance for credit losses								6
Cumulative Effect, Period of Adoption, Adjustment [Member] | Retained Earnings [Member] | ASU 2016-13 [Member]
Basis of Presentation [Abstract]
Cumulative effect of accounting change								$ (5)
Stock-based Compensation Awards [Member]
Basis of Presentation [Abstract]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding	0		0	0		0
CNA [Member]
Basis of Presentation [Abstract]
Subsidiary ownership percentage	89.60%			89.60%
Altium Packaging LLC [Member]
Basis of Presentation [Abstract]
Subsidiary ownership percentage	99.00%			99.00%

[1] On January 1, 2020, the Company adopted ASU 2016-13; see Note 1. The Unrealized OTTI Losses (Gains) column that tracked subsequent valuation changes on securities for which a credit loss had previously been recorded has been replaced with the Allowance for Credit Losses column. Prior period amounts were not adjusted for the adoption of this standard.