Securities - Schedule of Fair Value and Amortized Cost of Available-for-sale and Held-to-maturity Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	$ 2,177,178	$ 2,142,056
Available-for-sale, Gross Unrealized Gains	36,621	5,861
Available-for-sale, Gross Unrealized Losses	(4,600)	(33,788)
Available-for-sale, Estimated Fair Value	2,209,199	2,114,129
Held-to-maturity, Amortized Cost	852,824	1,020,934
Held-to-maturity, Gross Unrealized Gains	25,356	8,993
Held-to-maturity, Gross Unrealized Losses	(371)	(9,184)
Held-to-maturity securities, Fair value	877,809	1,020,743
Total securities, Amortized Cost	3,030,002	3,162,990
Total securities, Gross Unrealized Gains	61,977	14,854
Total securities, Gross Unrealized Losses	(4,971)	(42,972)
Total securities, Estimated Fair Value	3,087,008	3,134,872
US Treasury Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	29,817	19,882
Available-for-sale, Gross Unrealized Gains	61	3
Available-for-sale, Gross Unrealized Losses		(7)
Available-for-sale, Estimated Fair Value	29,878	19,878
U.S. Government Sponsored Entities and Agencies [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	116,508	142,852
Available-for-sale, Gross Unrealized Gains	4,012	556
Available-for-sale, Gross Unrealized Losses	(79)	(1,756)
Available-for-sale, Estimated Fair Value	120,441	141,652
Held-to-maturity, Amortized Cost	9,854	10,823
Held-to-maturity, Gross Unrealized Gains	53	6
Held-to-maturity, Gross Unrealized Losses	(46)	(329)
Held-to-maturity securities, Fair value	9,861	10,500
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Sponsored Entities and Agencies [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	1,663,942	1,585,864
Available-for-sale, Gross Unrealized Gains	21,469	2,912
Available-for-sale, Gross Unrealized Losses	(4,346)	(27,521)
Available-for-sale, Estimated Fair Value	1,681,065	1,561,255
Held-to-maturity, Amortized Cost	129,797	148,300
Held-to-maturity, Gross Unrealized Gains	1,273	204
Held-to-maturity, Gross Unrealized Losses	(155)	(4,170)
Held-to-maturity securities, Fair value	130,915	144,334
Commercial Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Sponsored Entities and Agencies [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	183,634	171,671
Available-for-sale, Gross Unrealized Gains	4,716	264
Available-for-sale, Gross Unrealized Losses	(45)	(2,963)
Available-for-sale, Estimated Fair Value	188,305	168,972
Obligations of State and Political Subdivisions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	145,593	184,057
Available-for-sale, Gross Unrealized Gains	5,771	2,039
Available-for-sale, Gross Unrealized Losses	(1)	(982)
Available-for-sale, Estimated Fair Value	151,363	185,114
Held-to-maturity, Amortized Cost	679,932	828,520
Held-to-maturity, Gross Unrealized Gains	22,244	8,771
Held-to-maturity, Gross Unrealized Losses	(170)	(4,012)
Held-to-maturity securities, Fair value	702,006	833,279
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	37,684	37,730
Available-for-sale, Gross Unrealized Gains	592	87
Available-for-sale, Gross Unrealized Losses	(129)	(559)
Available-for-sale, Estimated Fair Value	38,147	37,258
Held-to-maturity, Amortized Cost	33,241	33,291
Held-to-maturity, Gross Unrealized Gains	1,786	12
Held-to-maturity, Gross Unrealized Losses		(673)
Held-to-maturity securities, Fair value	$ 35,027	$ 32,630

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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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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