Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	$ 2,336,095	$ 2,068,246
Gross Unrealized Gains	34,556	14,436
Gross Unrealized Losses	(11,931)	(47,739)
Fair Value	2,358,720	2,034,943
U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	138,352	138,517
Gross Unrealized Gains	5,761	3,532
Gross Unrealized Losses	0	0
Fair Value	144,113	142,049
U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	7,345	8,721
Gross Unrealized Gains	58	55
Gross Unrealized Losses	(41)	(135)
Fair Value	7,362	8,641
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	297,454	341,702
Gross Unrealized Gains	7,745	4,332
Gross Unrealized Losses	(529)	(5,735)
Fair Value	304,670	340,299
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	203,251	243,954
Gross Unrealized Gains	5,696	3,141
Gross Unrealized Losses	(10)	(1,361)
Fair Value	208,937	245,734
Total obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	500,705	585,656
Gross Unrealized Gains	13,441	7,473
Gross Unrealized Losses	(539)	(7,096)
Fair Value	513,607	586,033
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	1,620,119	1,266,770
Gross Unrealized Gains	14,803	3,203
Gross Unrealized Losses	(5,350)	(34,368)
Fair Value	1,629,572	1,235,605
Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	37,324	37,332
Gross Unrealized Gains	39	77
Gross Unrealized Losses	(5,981)	(5,923)
Fair Value	31,382	31,486
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	32,250	31,250
Gross Unrealized Gains	454	96
Gross Unrealized Losses	(20)	(217)
Fair Value	$ 32,684	$ 31,129

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
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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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