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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.

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