Basis of Presentation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020		Dec. 31, 2019
Basis of Presentation [Abstract]
Allowance for credit losses	$ 49	[1]	$ 0
ASU 2016-13 [Member]
Basis of Presentation [Abstract]
Cumulative effect of accounting change			(5)
Allowance for credit losses			6
Retained Earnings [Member] | ASU 2016-13 [Member]
Basis of Presentation [Abstract]
Cumulative effect of accounting change			$ (5)
CNA Financial Corporation [Member]
Basis of Presentation [Abstract]
Subsidiary ownership percentage	89.00%
Diamond Offshore Drilling, Inc. [Member]
Basis of Presentation [Abstract]
Subsidiary ownership percentage	53.00%
Altium Packaging LLC [Member]
Basis of Presentation [Abstract]
Subsidiary ownership percentage	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.