Securities (Realized Gains and Losses Recognized in Income from Available-for-Sale Securities) (Parenthetical) (Detail) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Interest-Only Mortgage-Backed Securities
Investment Holdings
Net gains/losses on interest-only mortgage-backed securities	$ (3,000,000)	$ 11,000,000	$ (8,000,000)	$ 2,000,000

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