Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	$ 2,338,683	$ 2,035,126
Securities available-for-sale, Gross Unrealized Holding Gains	81,202	45,455
Securities available-for-sale, Gross Unrealized Holding Losses	(4,029)	(22,858)
Securities available-for-sale, Estimated Fair Value	2,415,856us-gaap_AvailableForSaleSecurities	2,057,723us-gaap_AvailableForSaleSecurities
U.S. Treasury Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	518
Securities available-for-sale, Gross Unrealized Holding Gains	2
Securities available-for-sale, Estimated Fair Value	520
Obligations of U.S. Government Sponsored-Enterprises and Agencies [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	128,919	136,416
Securities available-for-sale, Gross Unrealized Holding Gains	864	1,672
Securities available-for-sale, Gross Unrealized Holding Losses	(28)
Securities available-for-sale, Estimated Fair Value	129,755	138,088
Obligations of State and Political Subdivisions [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	1,107,795	974,608
Securities available-for-sale, Gross Unrealized Holding Gains	60,083	27,980
Securities available-for-sale, Gross Unrealized Holding Losses	(250)	(11,319)
Securities available-for-sale, Estimated Fair Value	1,167,628	991,269
Corporate Bonds and Other [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	95,864	105,490
Securities available-for-sale, Gross Unrealized Holding Gains	2,894	3,550
Securities available-for-sale, Estimated Fair Value	98,758	109,040
Residential Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	871,265	706,289
Securities available-for-sale, Gross Unrealized Holding Gains	16,804	12,253
Securities available-for-sale, Gross Unrealized Holding Losses	(2,858)	(7,922)
Securities available-for-sale, Estimated Fair Value	885,211	710,620
Commercial Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	134,322	112,323
Securities available-for-sale, Gross Unrealized Holding Gains	555
Securities available-for-sale, Gross Unrealized Holding Losses	(893)	(3,617)
Securities available-for-sale, Estimated Fair Value	$ 133,984	$ 108,706

Amount of accumulated pre-tax unrealized gain before deducting pre-tax unrealized losses on investments in available-for-sale securities impacting investments.

No definition available.

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Amount of accumulated pre-tax unrealized loss before deducting pre-tax unrealized gain on investments in available-for-sale securities impacting investments.

+ References

No definition available.

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- Definition

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

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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