Change in Accumulated Other Comprehensive Income - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 687,336	$ 580,471	$ 489,051
Other comprehensive income (loss) before reclassifications	2,889	(6,610)	(1,417)
Less: Amounts reclassified from accumulated other comprehensive income (loss)	(1,764)	(1,703)	(1,387)
Total other comprehensive income (loss)	1,125	(8,313)	(2,804)
Less: Reclassification due to the adoption of ASU No. 2018-02	0
Ending Balance	724,345	687,336	580,471
Net change in investment securities available for sale
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(8,194)	(1,887)	446
Other comprehensive income (loss) before reclassifications	3,073	(4,838)	(1,417)
Less: Amounts reclassified from accumulated other comprehensive income (loss)	(1,280)	(1,469)	(916)
Total other comprehensive income (loss)	1,793	(6,307)	(2,333)
Less: Reclassification due to the adoption of ASU No. 2018-02	(1,441)
Ending Balance	(7,842)	(8,194)	(1,887)
Net change in securities held to maturity
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	1,392	1,795	2,207
Other comprehensive income (loss) before reclassifications	0	0	0
Less: Amounts reclassified from accumulated other comprehensive income (loss)	(394)	(403)	(412)
Total other comprehensive income (loss)	(394)	(403)	(412)
Less: Reclassification due to the adoption of ASU No. 2018-02	225
Ending Balance	1,223	1,392	1,795
Net change in defined benefit plan
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	957	788	847
Other comprehensive income (loss) before reclassifications	0	0	0
Less: Amounts reclassified from accumulated other comprehensive income (loss)	(90)	169	(59)
Total other comprehensive income (loss)	(90)	169	(59)
Less: Reclassification due to the adoption of ASU No. 2018-02	(2)
Ending Balance	865	957	788
Net change in fair value of derivatives used for cash flow hedge
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(1,772)	0	0
Other comprehensive income (loss) before reclassifications	(184)	(1,772)	0
Less: Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0
Total other comprehensive income (loss)	(184)	(1,772)	0
Less: Reclassification due to the adoption of ASU No. 2018-02	(442)
Ending Balance	(2,398)	(1,772)	0
Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(7,617)	696	3,500
Less: Reclassification due to the adoption of ASU No. 2018-02	(1,660)
Ending Balance	$ (8,152)	$ (7,617)	$ 696