Securities - Schedule of Fair Value and Amortized Cost of Available-for-sale and Held-to-maturity Debt Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Available-for-sale, Amortized Cost	$ 1,843,332	$ 1,280,585
Available-for-sale, Gross Unrealized Gains	2,230	3,397
Available-for-sale, Gross Unrealized Losses	(48,991)	(22,117)
Available-for-sale, Estimated Fair Value	1,796,571	1,261,865
Held-to-maturity, Amortized Cost	1,019,746	1,009,500
Held-to-maturity, Gross Unrealized Gains	9,224	18,827
Held-to-maturity, Gross Unrealized Losses	(12,859)	(4,543)
Held-to-maturity securities, Fair value	1,016,111	1,023,784
Total securities, Amortized Cost	2,863,078	2,290,085
Total securities, Gross Unrealized Gains	11,454	22,224
Total securities, Gross Unrealized Losses	(61,850)	(26,660)
Total securities, Estimated Fair Value	2,812,682	2,285,649
U.S. Government Sponsored Entities and Agencies [Member]
Debt and Equity Securities, FV-NI [Line Items]
Available-for-sale, Amortized Cost	103,152	72,425
Available-for-sale, Gross Unrealized Gains		24
Available-for-sale, Gross Unrealized Losses	(2,470)	(606)
Available-for-sale, Estimated Fair Value	100,682	71,843
Held-to-maturity, Amortized Cost	11,877	11,465
Held-to-maturity, Gross Unrealized Losses	(390)	(325)
Held-to-maturity securities, Fair value	11,487	11,140
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Sponsored Entities and Agencies [Member]
Debt and Equity Securities, FV-NI [Line Items]
Available-for-sale, Amortized Cost	1,413,937	954,115
Available-for-sale, Gross Unrealized Gains	98	214
Available-for-sale, Gross Unrealized Losses	(40,618)	(19,407)
Available-for-sale, Estimated Fair Value	1,373,417	934,922
Held-to-maturity, Amortized Cost	161,090	170,025
Held-to-maturity, Gross Unrealized Gains	203	544
Held-to-maturity, Gross Unrealized Losses	(5,740)	(2,609)
Held-to-maturity securities, Fair value	155,553	167,960
Obligations of State and Political Subdivisions [Member]
Debt and Equity Securities, FV-NI [Line Items]
Available-for-sale, Amortized Cost	126,143	102,363
Available-for-sale, Gross Unrealized Gains	1,866	2,927
Available-for-sale, Gross Unrealized Losses	(1,210)	(460)
Available-for-sale, Estimated Fair Value	126,799	104,830
Held-to-maturity, Amortized Cost	813,456	794,655
Held-to-maturity, Gross Unrealized Gains	9,017	17,364
Held-to-maturity, Gross Unrealized Losses	(6,065)	(1,609)
Held-to-maturity securities, Fair value	816,408	810,410
Corporate Debt Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Available-for-sale, Amortized Cost	45,070	35,234
Available-for-sale, Gross Unrealized Gains	211	228
Available-for-sale, Gross Unrealized Losses	(168)	(59)
Available-for-sale, Estimated Fair Value	45,113	35,403
Held-to-maturity, Amortized Cost	33,323	33,355
Held-to-maturity, Gross Unrealized Gains	4	919
Held-to-maturity, Gross Unrealized Losses	(664)
Held-to-maturity securities, Fair value	32,663	34,274
US Treasury Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Available-for-sale, Amortized Cost	9,926	0
Available-for-sale, Gross Unrealized Gains	0
Available-for-sale, Gross Unrealized Losses	(13)
Available-for-sale, Estimated Fair Value	9,913
Commercial Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Sponsored Entities and Agencies [Member]
Debt and Equity Securities, FV-NI [Line Items]
Available-for-sale, Amortized Cost	145,104	116,448
Available-for-sale, Gross Unrealized Gains	55	4
Available-for-sale, Gross Unrealized Losses	(4,512)	(1,585)
Available-for-sale, Estimated Fair Value	$ 140,647	$ 114,867

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Securities amortized cost.

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