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