Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investments Securities with Unrealized Losses (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Schedule Of Fair Value And Gross Unrealized Losses [Abstract]
Unrealized Losses Total	$ 21

Available For Sale And Held To Maturity Securities Residential Mortgage Backed Non Agency Continuous Unrealized Loss Position Aggregate Losses Two

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