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	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Interest-Only Mortgage-Backed Securities
Investment Holdings [Line Items]
Net gains/losses on interest-only mortgage-backed securities	$ (9)	$ 1