Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 533,802
Gross Unrealized Gains	8,823
Gross Unrealized Losses	(781)
Estimated Fair Value	541,844us-gaap_AvailableForSaleSecurities
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	279,498
Gross Unrealized Gains	1,717
Estimated Fair Value	281,215	128,740
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	168,514
Gross Unrealized Gains	1,010
Gross Unrealized Losses	(62)
Estimated Fair Value	169,462	346,793
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	25,272
Gross Unrealized Gains	595
Gross Unrealized Losses	(567)
Estimated Fair Value	25,300	29,864
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,631
Gross Unrealized Gains	1,868
Gross Unrealized Losses	(19)
Estimated Fair Value	51,480	54,971
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,887
Gross Unrealized Gains	3,633
Gross Unrealized Losses	(133)
Estimated Fair Value	$ 14,387	$ 15,381

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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