Realized Gains and Losses Recognized in Income from Available-for-Sale Securities (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Interest-Only Mortgage Backed Securities
Investment Holdings [Line Items]
Net (losses) gains on interest-only mortgage-backed securities	$ (4)	$ (17)	$ 129

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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