Securities (Realized Gains and Losses Recognized in Income from Available-for-Sale Securities) (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Interest-Only Mortgage Backed Securities
Investment Holdings
Net (losses) gains on interest-only mortgage-backed securities	$ (4)	$ (4)	$ (17)

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