Securities - Schedule of Fair Value and Amortized Cost of Available-for-sale and Held-to-maturity Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	$ 2,369,416	$ 2,142,056
Available-for-sale, Gross Unrealized Gains	30,863	5,861
Available-for-sale, Gross Unrealized Losses	(6,721)	(33,788)
Available-for-sale, Estimated Fair Value	2,393,558	2,114,129
Held-to-maturity, Amortized Cost	851,753	1,020,934
Held-to-maturity, Gross Unrealized Gains	23,405	8,993
Held-to-maturity, Gross Unrealized Losses	(635)	(9,184)
Held-to-maturity securities, Fair value	874,523	1,020,743
Total securities, Amortized Cost	3,221,169	3,162,990
Total securities, Gross Unrealized Gains	54,268	14,854
Total securities, Gross Unrealized Losses	(7,356)	(42,972)
Total securities, Estimated Fair Value	3,268,081	3,134,872
US Treasury Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	32,790	19,882
Available-for-sale, Gross Unrealized Gains	47	3
Available-for-sale, Gross Unrealized Losses	(1)	(7)
Available-for-sale, Estimated Fair Value	32,836	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	157,088	142,852
Available-for-sale, Gross Unrealized Gains	2,862	556
Available-for-sale, Gross Unrealized Losses	(322)	(1,756)
Available-for-sale, Estimated Fair Value	159,628	141,652
Held-to-maturity, Amortized Cost	9,216	10,823
Held-to-maturity, Gross Unrealized Gains	30	6
Held-to-maturity, Gross Unrealized Losses	(116)	(329)
Held-to-maturity securities, Fair value	9,130	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	1,803,268	1,585,864
Available-for-sale, Gross Unrealized Gains	18,850	2,912
Available-for-sale, Gross Unrealized Losses	(6,131)	(27,521)
Available-for-sale, Estimated Fair Value	1,815,987	1,561,255
Held-to-maturity, Amortized Cost	122,937	148,300
Held-to-maturity, Gross Unrealized Gains	1,031	204
Held-to-maturity, Gross Unrealized Losses	(261)	(4,170)
Held-to-maturity securities, Fair value	123,707	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	187,268	171,671
Available-for-sale, Gross Unrealized Gains	3,270	264
Available-for-sale, Gross Unrealized Losses	(129)	(2,963)
Available-for-sale, Estimated Fair Value	190,409	168,972
Obligations of State and Political Subdivisions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	140,357	184,057
Available-for-sale, Gross Unrealized Gains	5,253	2,039
Available-for-sale, Gross Unrealized Losses	(1)	(982)
Available-for-sale, Estimated Fair Value	145,609	185,114
Held-to-maturity, Amortized Cost	686,376	828,520
Held-to-maturity, Gross Unrealized Gains	20,475	8,771
Held-to-maturity, Gross Unrealized Losses	(258)	(4,012)
Held-to-maturity securities, Fair value	706,593	833,279
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	48,645	37,730
Available-for-sale, Gross Unrealized Gains	581	87
Available-for-sale, Gross Unrealized Losses	(137)	(559)
Available-for-sale, Estimated Fair Value	49,089	37,258
Held-to-maturity, Amortized Cost	33,224	33,291
Held-to-maturity, Gross Unrealized Gains	1,869	12
Held-to-maturity, Gross Unrealized Losses		(673)
Held-to-maturity securities, Fair value	$ 35,093	$ 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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