Comprehensive Income (Loss) - Summary of Changes in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 550,044	$ 400,516	$ 379,438
Other comprehensive income (loss) before classifications	(882)	(324)	(216)
Amounts reclassified from accumulated other comprehensive income	(1,640)	(233)	(157)
Other comprehensive loss	(2,522)	(557)	(373)
Adjustment for implementation of ASU 2018-02	(420)
Balance at end of period	583,122	550,044	400,516
Unrealized Gains and Losses on Available- for-Sale Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(117)	1,154	1,531
Other comprehensive income (loss) before classifications	(903)	(1,038)	(220)
Amounts reclassified from accumulated other comprehensive income	(1,640)	(233)	(157)
Other comprehensive loss	(2,543)	(1,271)	(377)
Adjustment for implementation of ASU 2018-02	(572)
Balance at end of period	(3,232)	(117)	1,154
Unrealized Gains (Losses) on Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	672	0	0
Other comprehensive income (loss) before classifications	37	672	0
Amounts reclassified from accumulated other comprehensive income	0	0	0
Other comprehensive loss	37	672	0
Adjustment for implementation of ASU 2018-02	153
Balance at end of period	862	672	0
Pension Items
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	38	(4)	(8)
Other comprehensive income (loss) before classifications	(16)	42	4
Amounts reclassified from accumulated other comprehensive income	0	0	0
Other comprehensive loss	(16)	42	4
Adjustment for implementation of ASU 2018-02	(1)
Balance at end of period	21	38	(4)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	593	1,150	1,523
Other comprehensive loss	(2,522)	(557)	(373)
Balance at end of period	$ (2,349)	$ 593	$ 1,150