Investment Securities - Contractual Maturities of Debt Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013
Investments Debt And Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 123,929
Due after one year through five years, Amortized Cost	45,516
Due after five years through ten years, Amortized Cost	251,536
Due after ten years, Amortized Cost	392,406
Residential mortgage-backed securities, Amortized Cost	752,220
Total investment securities held to maturity, Amortized Cost	1,565,607
Due in one year, Fair Value	123,973
Due after one year through five years, Fair Value	47,965
Due after five years through ten years, Fair Value	276,705
Due after ten years, Fair Value	393,452
Residential mortgage-backed securities, Fair Value	772,617
Total investment securities held to maturity, Fair Value	$ 1,614,712

Amount of held-to-maturity debt securities at fair value maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year.

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Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized, and fair value hedge accounting adjustments, maturing in the fifth through tenth fiscal year following the latest fiscal year.

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Amount of held-to-maturity debt securities at fair value maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year.

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Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized, and fair value hedge accounting adjustments, maturing in the next fiscal year through the fifth fiscal year following the latest fiscal year.

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Amount of held-to-maturity debt securities at fair value maturing after the tenth fiscal year following the latest fiscal year.

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This item represents the total of all debt securities grouped by maturity dates, at fair value, which are categorized as held-to-maturity.

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Amount of held-to-maturity debt securities at fair value maturing in the next fiscal year following the latest fiscal year.

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Amount of held-to-maturity debt securities at fair value that do not have a single maturity date and which the entity has decided to disclose separately rather than allocating the fair value over several maturity groupings.

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