Securities - Cumulative Credit Portion of Impairments on Debt Securities (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2013
Asset Impairment Charges [Abstract]
Cumulative Impairments on Debt Securities, Beginning Balance	$ 4
Impairment recognized in earnings
Cumulative Impairments on Debt Securities, Ending Balance	$ 4