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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			$ 184,461
Amounts reclassified from accumulated other comprehensive loss	$ 118	$ 281	231	$ 343
Total other comprehensive income (loss)	(428)	1,240	(2,854)	1,601
Ending balance	189,848		189,848
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	368	2,307	3,185	2,008
Other comprehensive income before reclassifications	(546)	959	(3,085)	1,258
Amounts reclassified from accumulated other comprehensive loss	5		5
Total other comprehensive income (loss)	(541)	959	(3,080)	1,258
Reclassification of equity securities from accumulated other comprehensive loss			(278)
Ending balance	(173)	3,266	(173)	3,266
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(4,196)	(4,283)	(4,309)	(4,345)
Amounts reclassified from accumulated other comprehensive loss	113	281	226	343
Total other comprehensive income (loss)	113	281	226	343
Ending balance	(4,083)	(4,002)	(4,083)	(4,002)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(3,828)	(1,976)	(1,124)	(2,337)
Other comprehensive income before reclassifications	(546)	959	(3,085)	1,258
Amounts reclassified from accumulated other comprehensive loss	118	281	231	343
Total other comprehensive income (loss)	(428)	1,240	(2,854)	1,601
Reclassification of equity securities from accumulated other comprehensive loss			(278)
Ending balance	$ (4,256)	$ (736)	$ (4,256)	$ (736)