Securities - Available for Sale Securities Realized Gains and Losses (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Realized gains (losses)		$ 1
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Realized gains (losses)	1		1
Realized losses		$ 1