Securities - Fair Value of Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Total available for sale investment securities, Amortized Cost	$ 539,086	$ 513,602
Gross Unrealized Gains	585	2,502
Gross Unrealized Losses	(13,476)	(6,439)
Available-for-sale Securities, Fair Value	526,195	509,665
Held-to-maturity, Amortized Cost	209,767	200,448
Held-to-maturity, Gross Unrealized Gains	1,673	3,726
Held-to-maturity, Gross Unrealized Losses	(4,710)	(3,089)
Held-to-maturity, Fair Value	206,730	201,085
State and Municipal [Member]
Debt Securities, Available-for-sale [Line Items]
Total available for sale investment securities, Amortized Cost	136,732	148,045
Gross Unrealized Gains	300	2,189
Gross Unrealized Losses	(2,051)	(670)
Available-for-sale Securities, Fair Value	134,981	149,564
Held-to-maturity, Amortized Cost	190,079	179,836
Held-to-maturity, Gross Unrealized Gains	1,610	3,493
Held-to-maturity, Gross Unrealized Losses	(4,309)	(2,932)
Held-to-maturity, Fair Value	187,380	180,397
Federal Agency Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Total available for sale investment securities, Amortized Cost	168,382	132,871
Gross Unrealized Gains	112	45
Gross Unrealized Losses	(4,360)	(2,551)
Available-for-sale Securities, Fair Value	164,134	130,365
Held-to-maturity, Amortized Cost	5,409	5,734
Held-to-maturity, Gross Unrealized Gains	8	17
Held-to-maturity, Gross Unrealized Losses	(157)	(69)
Held-to-maturity, Fair Value	5,260	5,682
Federal Agency Mortgage-backed Pools [Member]
Debt Securities, Available-for-sale [Line Items]
Total available for sale investment securities, Amortized Cost	203,593	211,487
Gross Unrealized Gains	28	155
Gross Unrealized Losses	(6,626)	(2,985)
Available-for-sale Securities, Fair Value	196,995	208,657
Held-to-maturity, Amortized Cost	14,279	14,878
Held-to-maturity, Gross Unrealized Gains	55	216
Held-to-maturity, Gross Unrealized Losses	(244)	(88)
Held-to-maturity, Fair Value	14,090	15,006
U.S. Treasury and Federal Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Total available for sale investment securities, Amortized Cost	24,654	19,277
Gross Unrealized Losses	(435)	(225)
Available-for-sale Securities, Fair Value	24,219	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	5,725	272
Gross Unrealized Gains	145	113
Gross Unrealized Losses	(4)
Available-for-sale Securities, Fair Value	$ 5,866	$ 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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