Change in Accumulated Other Comprehensive Income - Components of Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			$ 687,336	$ 580,471
Ending Balance	$ 722,623	$ 617,196	722,623	617,196
Net change in investment securities available for sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(7,128)	8,496	(8,194)	(1,887)
Other comprehensive income (loss) before reclassifications	3,241	4,683	4,513	15,255
Less: Amounts reclassified from accumulated other comprehensive loss	(455)	(338)	(661)	(527)
Net current-period other comprehensive income (loss)	2,786	4,345	3,852	14,728
Ending Balance	(4,342)	12,841	(4,342)	12,841
Net change in securities held to maturity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	1,291	1,692	1,392	1,795
Other comprehensive income (loss) before reclassifications	0	0	0	0
Less: Amounts reclassified from accumulated other comprehensive loss	(97)	(100)	(198)	(203)
Net current-period other comprehensive income (loss)	(97)	(100)	(198)	(203)
Ending Balance	1,194	1,592	1,194	1,592
Net change in defined benefit plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	934	1,266	957	788
Other comprehensive income (loss) before reclassifications	0	0	0	0
Less: Amounts reclassified from accumulated other comprehensive loss	(22)	(22)	(45)	456
Net current-period other comprehensive income (loss)	(22)	(22)	(45)	456
Ending Balance	912	1,244	912	1,244
Net change in fair value of derivative used for cash flow hedge
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(1,884)	0	(1,772)	0
Other comprehensive income (loss) before reclassifications	262	0	150	0
Less: Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net current-period other comprehensive income (loss)	262	0	150	0
Ending Balance	(1,622)	0	(1,622)	0
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(6,787)	11,454	(7,617)	696
Other comprehensive income (loss) before reclassifications	3,503	4,683	4,663	15,255
Less: Amounts reclassified from accumulated other comprehensive loss	(574)	(460)	(904)	(274)
Net current-period other comprehensive income (loss)	2,929	4,223	3,759	14,981
Ending Balance	$ (3,858)	$ 15,677	$ (3,858)	$ 15,677