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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 698,463	$ 597,864	$ 623,739	$ 583,122
Cumulative adjustment for adoption of ASU 2016-01					$ 125
Adjusted equity balance					623,864		$ 583,122
Other comprehensive income (loss) before classifications	1,402	(2,013)	11,048	(9,099)
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Net current period other comprehensive income (loss)	1,402	(2,013)	11,048	(9,099)
Balance at end of period	713,204	607,555	713,204	607,555
Unrealized Gains (Losses) on Available for Sale Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	1,357	(12,697)	(8,782)	(3,232)
Cumulative adjustment for adoption of ASU 2016-01						$ 0		$ (2,043)
Adjusted equity balance						(8,782)		(5,275)
Other comprehensive income (loss) before classifications	1,480	(2,019)	11,619	(9,441)
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Net current period other comprehensive income (loss)	1,480	(2,019)	11,619	(9,441)
Balance at end of period	2,837	(14,716)	2,837	(14,716)
Unrealized Gains on Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	410	1,198	903	862
Cumulative adjustment for adoption of ASU 2016-01						0		0
Adjusted equity balance						903		862
Other comprehensive income (loss) before classifications	(78)	6	(571)	342
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Net current period other comprehensive income (loss)	(78)	6	(571)	342
Balance at end of period	332	1,204	332	1,204
Pension Items
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	41	21	41	21
Cumulative adjustment for adoption of ASU 2016-01						0		0
Adjusted equity balance						41		21
Other comprehensive income (loss) before classifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Net current period other comprehensive income (loss)	0	0	0	0
Balance at end of period	41	21	41	21
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	1,808	(11,478)	(7,838)	(2,349)
Cumulative adjustment for adoption of ASU 2016-01						0	(2,043)	(2,043)
Adjusted equity balance					$ (7,838)	$ (7,838)	$ (4,392)	$ (4,392)
Balance at end of period	$ 3,210	$ (13,491)	$ 3,210	$ (13,491)