Investment Securities - Amortized Cost, Securities Available for Sale (Detail) $ in Thousands	Mar. 31, 2018 USD ($) security Security	Dec. 31, 2017 USD ($) security Security
Schedule of Available-for-sale Securities
Amortized Cost	$ 1,890,335	$ 1,511,034
Gross Unrealized Gains	2,540	5,029
Gross Unrealized Losses	(49,361)	(22,158)
Fair Value	$ 1,843,514	$ 1,493,905
Number of available for sale securities (in security) \| security	548	327
U.S. Treasury securities
Schedule of Available-for-sale Securities
Amortized Cost	$ 50,992	$ 50,997
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2,086)	(1,355)
Fair Value	48,906	49,642
U.S. government agency securities
Schedule of Available-for-sale Securities
Amortized Cost	41,140	42,384
Gross Unrealized Gains	132	158
Gross Unrealized Losses	(682)	(37)
Fair Value	40,590	42,505
Obligations of states and state agencies
Schedule of Available-for-sale Securities
Amortized Cost	114,081	38,435
Gross Unrealized Gains	55	158
Gross Unrealized Losses	(1,853)	(374)
Fair Value	112,283	38,219
Municipal bonds
Schedule of Available-for-sale Securities
Amortized Cost	109,876	74,752
Gross Unrealized Gains	321	477
Gross Unrealized Losses	(1,306)	(564)
Fair Value	108,891	74,665
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities
Amortized Cost	223,957	113,187
Gross Unrealized Gains	376	635
Gross Unrealized Losses	(3,159)	(938)
Fair Value	221,174	112,884
Residential mortgage-backed securities
Schedule of Available-for-sale Securities
Amortized Cost	1,514,862	1,239,534
Gross Unrealized Gains	1,708	2,423
Gross Unrealized Losses	(42,495)	(18,662)
Fair Value	1,474,075	1,223,295
Trust preferred securities
Schedule of Available-for-sale Securities
Amortized Cost	3,628	3,726
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(445)	(512)
Fair Value	3,183	3,214
Corporate and other debt securities
Schedule of Available-for-sale Securities
Amortized Cost	55,756	50,701
Gross Unrealized Gains	324	623
Gross Unrealized Losses	(494)	(160)
Fair Value	$ 55,586	51,164
Equity securities
Schedule of Available-for-sale Securities
Amortized Cost		10,505
Gross Unrealized Gains		1,190
Gross Unrealized Losses		(494)
Fair Value		$ 11,201
Pooled Trust Preferred Securities
Schedule of Available-for-sale Securities
Available-For-Sale Securities, Number Of Securities \| Security	1	1

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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