Securities (Realized Gains and Losses Recognized in Income from Available-for-Sale Securities) (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Interest-Only Mortgage-Backed Securities
Investment Holdings
Net gains/losses on interest-only mortgage-backed securities	$ (7)	$ (8)	$ (4)

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