Other Comprehensive Income - Changes in Each Component of Accumulated Other Comprehensive Income (Loss), Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 328,167	$ 312,057	$ 330,126	$ 298,898
Total other comprehensive income	3,766	5,092	6,780	10,061
Ending balance	336,613	324,212	336,613	324,212
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	15,840	7,200	12,883	2,347
Other comprehensive income before reclassifications	3,709	4,984	6,666	9,840
Amounts reclassified from accumulated other comprehensive income (loss)		(8)		(11)
Total other comprehensive income	3,709	4,976	6,666	9,829
Ending balance	19,549	12,176	19,549	12,176
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(5,952)	(3,683)	(6,009)	(3,799)
Amounts reclassified from accumulated other comprehensive income (loss)	57	116	114	232
Total other comprehensive income	57	116	114	232
Ending balance	(5,895)	(3,567)	(5,895)	(3,567)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	9,888	3,517	6,874	(1,452)
Other comprehensive income before reclassifications	3,709	4,984	6,666	9,840
Amounts reclassified from accumulated other comprehensive income (loss)	57	108	114	221
Total other comprehensive income	3,766	5,092	6,780	10,061
Ending balance	$ 13,654	$ 8,609	$ 13,654	$ 8,609