Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 525,397	$ 509,353
Gross Unrealized Gains	4,813	7,957
Gross Unrealized Losses	(814)	(1,120)
Estimated Fair Value	529,396	516,190
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	279,414	248,767
Gross Unrealized Gains	1,571	404
Gross Unrealized Losses		(178)
Estimated Fair Value	280,985	248,993
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	165,090	171,641
Gross Unrealized Gains	1,055	983
Gross Unrealized Losses	(28)	(175)
Estimated Fair Value	166,117	172,449
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,793	26,441
Gross Unrealized Gains	526	602
Gross Unrealized Losses	(549)	(586)
Estimated Fair Value	23,770	26,457
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	47,560	51,706
Gross Unrealized Gains	1,457	1,716
Gross Unrealized Losses	(40)	(49)
Estimated Fair Value	48,977	53,373
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,540	10,798
Gross Unrealized Gains	204	4,252
Gross Unrealized Losses	(197)	(132)
Estimated Fair Value	$ 9,547	$ 14,918

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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