Other Comprehensive Income - Changes in Each Component of Accumulated Other Comprehensive Income (Loss), Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 330,126	$ 298,898
Total other comprehensive income	3,014	4,969
Ending balance	328,167	312,057
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	12,883	2,347
Other comprehensive income before reclassifications	2,957	4,850
Amounts reclassified from accumulated other comprehensive income (loss)		3
Total other comprehensive income	2,957	4,853
Ending balance	15,840	7,200
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(6,009)	(3,799)
Amounts reclassified from accumulated other comprehensive income (loss)	57	116
Total other comprehensive income	57	116
Ending balance	(5,952)	(3,683)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	6,874	(1,452)
Other comprehensive income before reclassifications	2,957	4,850
Amounts reclassified from accumulated other comprehensive income (loss)	57	119
Total other comprehensive income	3,014	4,969
Ending balance	$ 9,888	$ 3,517