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Investment Securities (Gains (Losses) on Sales of Securities Available for Sale) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Investment Securities Disclosure [Abstract]      
Gross Gains $ 57 $ 38 $ 24
Gross Losses $ (57) (31) (8)
Net Gains   7 16
Tax Expense   $ 2 $ 6