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Summary of Significant Accounting Policies Held-to-Maturities Securities, Gains (Losses) (Details) - USD ($)
$ in Millions
12 Months Ended
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