Investment Securities - Amortized Cost, Securities Available for Sale (Detail) $ in Thousands	Sep. 30, 2017 USD ($) Security	Dec. 31, 2016 USD ($) Security
Schedule of Available-for-sale Securities
Amortized Cost	$ 1,458,572	$ 1,315,723
Gross Unrealized Gains	6,066	4,066
Gross Unrealized Losses	(16,901)	(22,416)
Fair Value	1,447,737	1,297,373
U.S. Treasury securities
Schedule of Available-for-sale Securities
Amortized Cost	51,003	51,020
Gross Unrealized Gains	6	6
Gross Unrealized Losses	(861)	(1,435)
Fair Value	50,148	49,591
U.S. government agency securities
Schedule of Available-for-sale Securities
Amortized Cost	43,896	22,815
Gross Unrealized Gains	342	232
Gross Unrealized Losses	(8)	(6)
Fair Value	44,230	23,041
Obligations of states and state agencies
Schedule of Available-for-sale Securities
Amortized Cost	38,847	40,696
Gross Unrealized Gains	367	70
Gross Unrealized Losses	(204)	(424)
Fair Value	39,010	40,342
Municipal bonds
Schedule of Available-for-sale Securities
Amortized Cost	79,452	80,045
Gross Unrealized Gains	473	147
Gross Unrealized Losses	(533)	(767)
Fair Value	79,392	79,425
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities
Amortized Cost	118,299	120,741
Gross Unrealized Gains	840	217
Gross Unrealized Losses	(737)	(1,191)
Fair Value	118,402	119,767
Residential mortgage-backed securities
Schedule of Available-for-sale Securities
Amortized Cost	1,171,510	1,029,827
Gross Unrealized Gains	3,230	2,061
Gross Unrealized Losses	(13,583)	(16,346)
Fair Value	1,161,157	1,015,542
Trust preferred securities
Schedule of Available-for-sale Securities
Amortized Cost	6,532	10,164
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,139)	(2,155)
Fair Value	5,393	8,009
Corporate and other debt securities
Schedule of Available-for-sale Securities
Amortized Cost	56,827	60,651
Gross Unrealized Gains	719	436
Gross Unrealized Losses	(119)	(522)
Fair Value	57,427	60,565
Equity securities
Schedule of Available-for-sale Securities
Amortized Cost	10,505	20,505
Gross Unrealized Gains	929	1,114
Gross Unrealized Losses	(454)	(761)
Fair Value	$ 10,980	$ 20,858
Pooled Trust Preferred Securities
Schedule of Available-for-sale Securities
Number of available for sale securities (in security) \| Security	2	2

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
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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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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-Publisher FASB
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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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