Securities (Realized Gains and Losses Recognized in Income from Available-for-Sale Securities) (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Interest-Only Mortgage-Backed Securities
Investment Holdings
Net gains/losses on interest-only mortgage-backed securities	$ (2)	$ (3)	$ (1)	$ (8)

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