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		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Interest-Only Mortgage-Backed Securities
Investment Holdings [Line Items]
Net gains on interest-only mortgage-backed securities	$ 1	$ 40	$ 9	$ 121