Realized Gains and Losses Recognized in Income from Available-for-Sale Securities (Parenthetical) (Detail) (Interest-Only-Strip [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Interest-Only-Strip [Member]
Investment Holdings [Line Items]
Net gains on interest-only mortgage-backed securities	$ 8	$ 56	$ 7	$ 81