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Summary of Significant Accounting Policies Held-to-Maturities Securities, Gains (Losses) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract] |  |  |
Debt Securities, Held-to-maturity, Sold, Realized Gain (Loss) | $ 0 | $ 0 | $ 0

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- Definition Amount of gain (loss) on sale of investment in debt security measured at amortized cost (held-to-maturity), including other-than-temporary impairment (OTTI). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27405-111563 + Details
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- References No definition available. + Details
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