Comprehensive Income - Summary of Changes in Components of Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 597,864	$ 567,545	$ 583,122	$ 550,044
Cumulative adjustment for adoption of ASU 2016-01					$ 0
Adjusted equity balance					583,122
Other comprehensive (loss) income before classifications	(2,013)	172	(9,099)	1,828
Amounts reclassified from accumulated other comprehensive income	0	0	0	(1,640)
Other comprehensive income (loss)	(2,013)	172	(9,099)	188
Balance at end of period	607,555	577,081	607,555	577,081
Unrealized Losses on Available for Sale Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(12,697)	6	(3,232)	(117)
Cumulative adjustment for adoption of ASU 2016-01					(2,043)	$ 0
Adjusted equity balance					(5,275)	(117)
Other comprehensive (loss) income before classifications	(2,019)	170	(9,441)	1,933
Amounts reclassified from accumulated other comprehensive income	0	0	0	(1,640)
Other comprehensive income (loss)	(2,019)	170	(9,441)	293
Balance at end of period	(14,716)	176	(14,716)	176
Unrealized Gains on Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	1,198	565	862	672
Cumulative adjustment for adoption of ASU 2016-01					0	0
Adjusted equity balance					862	672
Other comprehensive (loss) income before classifications	6	2	342	(105)
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Other comprehensive income (loss)	6	2	342	(105)
Balance at end of period	1,204	567	1,204	567
Pension Items
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	21	38	21	38
Cumulative adjustment for adoption of ASU 2016-01					0	0
Adjusted equity balance					21	38
Other comprehensive (loss) income before classifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Other comprehensive income (loss)	0	0	0	0
Balance at end of period	21	38	21	38
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(11,478)	609	(2,349)	593
Cumulative adjustment for adoption of ASU 2016-01					(2,043)	0
Adjusted equity balance					$ (4,392)	$ 593
Balance at end of period	$ (13,491)	$ 781	$ (13,491)	$ 781