Securities - Fair Value of Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total available for sale investment securities, Amortized Cost	$ 519,798	$ 513,602
Gross Unrealized Gains	497	2,502
Gross Unrealized Losses	(12,559)	(6,439)
Available-for-sale Securities, Fair Value	507,736	509,665
Held-to-maturity, Amortized Cost	206,689	200,448
Held-to-maturity, Gross Unrealized Gains	1,895	3,726
Held-to-maturity, Gross Unrealized Losses	(4,688)	(3,089)
Held-to-maturity, Fair Value	203,896	201,085
State and Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available for sale investment securities, Amortized Cost	135,021	148,045
Gross Unrealized Gains	298	2,189
Gross Unrealized Losses	(1,948)	(670)
Available-for-sale Securities, Fair Value	133,371	149,564
Held-to-maturity, Amortized Cost	186,529	179,836
Held-to-maturity, Gross Unrealized Gains	1,793	3,493
Held-to-maturity, Gross Unrealized Losses	(4,339)	(2,932)
Held-to-maturity, Fair Value	183,983	180,397
Federal Agency Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available for sale investment securities, Amortized Cost	137,443	132,871
Gross Unrealized Gains	28	45
Gross Unrealized Losses	(4,063)	(2,551)
Available-for-sale Securities, Fair Value	133,408	130,365
Held-to-maturity, Amortized Cost	5,566	5,734
Held-to-maturity, Gross Unrealized Gains	11	17
Held-to-maturity, Gross Unrealized Losses	(137)	(69)
Held-to-maturity, Fair Value	5,440	5,682
Federal Agency Mortgage-backed Pools [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available for sale investment securities, Amortized Cost	218,274	211,487
Gross Unrealized Gains	40	155
Gross Unrealized Losses	(6,141)	(2,985)
Available-for-sale Securities, Fair Value	212,173	208,657
Held-to-maturity, Amortized Cost	14,594	14,878
Held-to-maturity, Gross Unrealized Gains	91	216
Held-to-maturity, Gross Unrealized Losses	(212)	(88)
Held-to-maturity, Fair Value	14,473	15,006
U.S. Treasury and Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available for sale investment securities, Amortized Cost	24,665	19,277
Gross Unrealized Gains	1
Gross Unrealized Losses	(382)	(225)
Available-for-sale Securities, Fair Value	24,284	19,052
Private Labeled Mortgage-backed Pools [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available for sale investment securities, Amortized Cost	4,135	1,650
Gross Unrealized Losses	(25)	(8)
Available-for-sale Securities, Fair Value	4,110	1,642
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available for sale investment securities, Amortized Cost	260	272
Gross Unrealized Gains	130	113
Available-for-sale Securities, Fair Value	$ 390	$ 385

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

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