Securities - Fair Value of Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Total available for sale investment securities, Amortized Cost	$ 558,786	$ 513,602
Gross Unrealized Gains	176	2,502
Gross Unrealized Losses	(16,657)	(6,439)
Available-for-sale Securities, Fair Value	542,305	509,665
Held-to-maturity, Amortized Cost	223,848	200,448
Held-to-maturity, Gross Unrealized Gains	1,092	3,726
Held-to-maturity, Gross Unrealized Losses	(5,782)	(3,089)
Held-to-maturity, Fair Value	219,158	201,085
State and Municipal [Member]
Debt Securities, Available-for-sale [Line Items]
Total available for sale investment securities, Amortized Cost	159,812	148,045
Gross Unrealized Gains	57	2,189
Gross Unrealized Losses	(3,194)	(670)
Available-for-sale Securities, Fair Value	156,675	149,564
Held-to-maturity, Amortized Cost	204,619	179,836
Held-to-maturity, Gross Unrealized Gains	1,062	3,493
Held-to-maturity, Gross Unrealized Losses	(5,257)	(2,932)
Held-to-maturity, Fair Value	200,424	180,397
Federal Agency Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Total available for sale investment securities, Amortized Cost	178,069	132,871
Gross Unrealized Gains		45
Gross Unrealized Losses	(5,398)	(2,551)
Available-for-sale Securities, Fair Value	172,671	130,365
Held-to-maturity, Amortized Cost	5,262	5,734
Held-to-maturity, Gross Unrealized Gains	3	17
Held-to-maturity, Gross Unrealized Losses	(182)	(69)
Held-to-maturity, Fair Value	5,083	5,682
Federal Agency Mortgage-backed Pools [Member]
Debt Securities, Available-for-sale [Line Items]
Total available for sale investment securities, Amortized Cost	193,461	211,487
Gross Unrealized Gains	16	155
Gross Unrealized Losses	(7,502)	(2,985)
Available-for-sale Securities, Fair Value	185,975	208,657
Held-to-maturity, Amortized Cost	13,967	14,878
Held-to-maturity, Gross Unrealized Gains	27	216
Held-to-maturity, Gross Unrealized Losses	(343)	(88)
Held-to-maturity, Fair Value	13,651	15,006
U.S. Treasury and Federal Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Total available for sale investment securities, Amortized Cost	17,036	19,277
Gross Unrealized Losses	(464)	(225)
Available-for-sale Securities, Fair Value	16,572	19,052
Private Labeled Mortgage-backed Pools [Member]
Debt Securities, Available-for-sale [Line Items]
Total available for sale investment securities, Amortized Cost		1,650
Gross Unrealized Losses		(8)
Available-for-sale Securities, Fair Value		1,642
Corporate Notes [Member]
Debt Securities, Available-for-sale [Line Items]
Total available for sale investment securities, Amortized Cost	10,408	272
Gross Unrealized Gains	103	113
Gross Unrealized Losses	(99)
Available-for-sale Securities, Fair Value	$ 10,412	$ 385

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

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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 before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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

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