Other Comprehensive Income - Changes in Each Component of Accumulated Other Comprehensive Income (Loss), Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			$ 137,616
Amounts reclassified from accumulated other comprehensive loss	$ 288	$ 43	631	$ 152
Total other comprehensive income (loss)	339	(753)	1,940	1,666
Ending balance	181,981		181,981
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	3,266	5,863	2,008	3,554
Other comprehensive income before reclassifications	51	(796)	1,309	1,514
Amounts reclassified from accumulated other comprehensive loss	6	(12)	6	(13)
Total other comprehensive income (loss)	57	(808)	1,315	1,501
Ending balance	3,323	5,055	3,323	5,055
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(4,002)	(3,939)	(4,345)	(4,049)
Amounts reclassified from accumulated other comprehensive loss	282	55	625	165
Total other comprehensive income (loss)	282	55	625	165
Ending balance	(3,720)	(3,884)	(3,720)	(3,884)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(736)	1,924	(2,337)	(495)
Other comprehensive income before reclassifications	51	(796)	1,309	1,514
Amounts reclassified from accumulated other comprehensive loss	288	43	631	152
Total other comprehensive income (loss)	339	(753)	1,940	1,666
Ending balance	$ (397)	$ 1,171	$ (397)	$ 1,171