Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	$ 2,598,678	$ 2,338,683
Securities available-for-sale, Gross Unrealized Holding Gains	92,762	81,202
Securities available-for-sale, Gross Unrealized Holding Losses	(2,110)	(4,029)
Securities available-for-sale, Estimated Fair Value	2,689,330us-gaap_AvailableForSaleSecurities	2,415,856us-gaap_AvailableForSaleSecurities	2,163,017us-gaap_AvailableForSaleSecurities
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	10,899	518
Securities available-for-sale, Gross Unrealized Holding Gains	84	2
Securities available-for-sale, Estimated Fair Value	10,983	520
Obligations of U.S. Government Sponsored-Enterprises and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	173,777	128,919
Securities available-for-sale, Gross Unrealized Holding Gains	1,340	864
Securities available-for-sale, Gross Unrealized Holding Losses		(28)
Securities available-for-sale, Estimated Fair Value	175,117	129,755
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	1,238,354	1,107,795
Securities available-for-sale, Gross Unrealized Holding Gains	65,922	60,083
Securities available-for-sale, Gross Unrealized Holding Losses	(784)	(250)
Securities available-for-sale, Estimated Fair Value	1,303,492	1,167,628
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	95,044	95,864
Securities available-for-sale, Gross Unrealized Holding Gains	3,089	2,894
Securities available-for-sale, Estimated Fair Value	98,133	98,758
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	900,831	871,265
Securities available-for-sale, Gross Unrealized Holding Gains	20,752	16,804
Securities available-for-sale, Gross Unrealized Holding Losses	(1,217)	(2,858)
Securities available-for-sale, Estimated Fair Value	920,366	885,211
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	179,773	134,322
Securities available-for-sale, Gross Unrealized Holding Gains	1,575	555
Securities available-for-sale, Gross Unrealized Holding Losses	(109)	(893)
Securities available-for-sale, Estimated Fair Value	$ 181,239	$ 133,984

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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