Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities Available for Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	$ 1,796,404	$ 1,511,034
Gross Unrealized Gains	1,995	5,029
Gross Unrealized Losses	(48,855)	(22,158)
Fair Value	1,749,544	1,493,905
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	50,975	50,997
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,669)	(1,355)
Fair Value	49,306	49,642
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	36,844	42,384
Gross Unrealized Gains	71	158
Gross Unrealized Losses	(638)	(37)
Fair Value	36,277	42,505
Obligations of states and state agencies
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	100,777	38,435
Gross Unrealized Gains	18	158
Gross Unrealized Losses	(3,682)	(374)
Fair Value	97,113	38,219
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	101,207	74,752
Gross Unrealized Gains	209	477
Gross Unrealized Losses	(1,437)	(564)
Fair Value	99,979	74,665
Total obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	201,984	113,187
Gross Unrealized Gains	227	635
Gross Unrealized Losses	(5,119)	(938)
Fair Value	197,092	112,884
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	1,469,059	1,239,534
Gross Unrealized Gains	1,484	2,423
Gross Unrealized Losses	(40,761)	(18,662)
Fair Value	1,429,782	1,223,295
Trust preferred securities
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale		3,726
Gross Unrealized Gains		0
Gross Unrealized Losses		(512)
Fair Value		3,214
Corporate and other debt securities
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale	37,542	50,701
Gross Unrealized Gains	213	623
Gross Unrealized Losses	(668)	(160)
Fair Value	$ 37,087	51,164
Equity securities
Debt Securities, Available-for-sale [Line Items]
Total investment securities available for sale		10,505
Gross Unrealized Gains		1,190
Gross Unrealized Losses		(494)
Fair Value		$ 11,201

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).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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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).

+ References

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