Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	$ 2,068,246	$ 1,842,691
Gross Unrealized Gains	14,436	25,419
Gross Unrealized Losses	(47,739)	(30,490)
Fair Value	2,034,943	1,837,620
U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	138,517	138,676
Gross Unrealized Gains	3,532	6,581
Gross Unrealized Losses	0	0
Fair Value	142,049	145,257
U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	8,721	9,859
Gross Unrealized Gains	55	122
Gross Unrealized Losses	(135)	0
Fair Value	8,641	9,981
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	341,702	244,272
Gross Unrealized Gains	4,332	7,083
Gross Unrealized Losses	(5,735)	(1,653)
Fair Value	340,299	249,702
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	243,954	221,606
Gross Unrealized Gains	3,141	6,199
Gross Unrealized Losses	(1,361)	(28)
Fair Value	245,734	227,777
Total obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	585,656	465,878
Gross Unrealized Gains	7,473	13,282
Gross Unrealized Losses	(7,096)	(1,681)
Fair Value	586,033	477,479
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	1,266,770	1,131,945
Gross Unrealized Gains	3,203	4,842
Gross Unrealized Losses	(34,368)	(18,743)
Fair Value	1,235,605	1,118,044
Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	37,332	49,824
Gross Unrealized Gains	77	60
Gross Unrealized Losses	(5,923)	(9,796)
Fair Value	31,486	40,088
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	31,250	46,509
Gross Unrealized Gains	96	532
Gross Unrealized Losses	(217)	(270)
Fair Value	$ 31,129	$ 46,771

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

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-Publisher FASB
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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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