Fair Value Measurements - Assets Measured at Fair Value on Nonrecurring Basis (Parenthetical) (Detail) (Nonrecurring [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Nonrecurring [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Transfer of commercial and consumer loans from held-for-sale to held-to-maturity, current fair value	$ 10	$ 2	$ 11