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Fair Value Measurements - Assets and Liabilities Measured at Recurring Fair Value (Parenthetical) (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis    
Federal Home Loan Bank Stock $ 96 $ 184
Federal Reserve Bank Stock 477 366
DTCC Stock $ 2 $ 2