Comprehensive Income - Summary of Changes in Components of Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 583,122	$ 550,044
Cumulative adjustment for adoption of ASU 2016-01			$ 0
Adjusted equity balance			583,122
Other comprehensive income (loss) before classifications	(5,449)	748
Amounts reclassified from accumulated other comprehensive income	0	(1,649)
Other comprehensive income (loss)	(5,449)	(901)
Balance at end of period	588,648	557,642
Unrealized Losses on Available for Sale Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(3,232)	(117)
Cumulative adjustment for adoption of ASU 2016-01			(2,043)	$ 0
Adjusted equity balance			(5,275)	(117)
Other comprehensive income (loss) before classifications	(5,732)	734
Amounts reclassified from accumulated other comprehensive income	0	(1,649)
Other comprehensive income (loss)	(5,732)	(915)
Balance at end of period	(11,007)	(1,032)
Unrealized Gains on Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	862	672
Cumulative adjustment for adoption of ASU 2016-01			0	0
Adjusted equity balance			862	672
Other comprehensive income (loss) before classifications	283	14
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income (loss)	283	14
Balance at end of period	1,145	686
Pension Items
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	21	38
Cumulative adjustment for adoption of ASU 2016-01			0	0
Adjusted equity balance			21	38
Other comprehensive income (loss) before classifications	0	0
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income (loss)	0	0
Balance at end of period	21	38
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(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	$ (9,841)	$ (308)