Comprehensive Income - Summary of Changes in Components of Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 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	$ 681,343	$ 588,648	$ 623,739	$ 583,122
Other comprehensive income (loss) before classifications	5,497	(1,637)	9,646	(7,086)
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Net current period other comprehensive income (loss)	5,497	(1,637)	9,646	(7,086)
Cumulative adjustment for adoption of ASU 2016-01					$ 125
Adjusted equity balance					623,864		$ 583,122
Balance at end of period	698,463	597,864	698,463	597,864
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	(4,419)	(11,007)	(8,782)	(3,232)
Other comprehensive income (loss) before classifications	5,776	(1,690)	10,139	(7,422)
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Net current period other comprehensive income (loss)	5,776	(1,690)	10,139	(7,422)
Cumulative adjustment for adoption of ASU 2016-01						$ 0		$ (2,043)
Adjusted equity balance						(8,782)		(5,275)
Balance at end of period	1,357	(12,697)	1,357	(12,697)
Unrealized Gains on Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	689	1,145	903	862
Other comprehensive income (loss) before classifications	(279)	53	(493)	336
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Net current period other comprehensive income (loss)	(279)	53	(493)	336
Cumulative adjustment for adoption of ASU 2016-01						0		0
Adjusted equity balance						903		862
Balance at end of period	410	1,198	410	1,198
Pension Items
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	41	21	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
Cumulative adjustment for adoption of ASU 2016-01						0		0
Adjusted equity balance						41		21
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	(3,689)	(9,841)	(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	$ 1,808	$ (11,478)	$ 1,808	$ (11,478)