Other Comprehensive Income - Changes in Each Component of Accumulated Other Comprehensive Loss, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 137,616
Amounts reclassified from accumulated other comprehensive loss	62	$ 52
Total other comprehensive income	361	1,313
Ending balance	174,737
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	2,008	3,554
Other comprehensive income before reclassifications	299	1,261
Amounts reclassified from accumulated other comprehensive loss		(3)
Total other comprehensive income	299	1,258
Ending balance	2,307	4,812
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(4,345)	(4,049)
Amounts reclassified from accumulated other comprehensive loss	62	55
Total other comprehensive income	62	55
Ending balance	(4,283)	(3,994)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(2,337)	(495)
Other comprehensive income before reclassifications	299	1,261
Amounts reclassified from accumulated other comprehensive loss	62	52
Total other comprehensive income	361	1,313
Ending balance	$ (1,976)	$ 818