Securities (Realized Gains and Losses Recognized in Income from Available-for-Sale Securities) (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Interest-Only Mortgage-Backed Securities
Investment Holdings
Net gains/losses on interest-only mortgage-backed securities	$ 1	$ (6)

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