Changes in Accumulated Other Comprehensive Income (Loss) - Components of Accumulated Other Comprehensive Income (Loss) Net of Taxes (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	[1]	$ 114,554
Change in net unrealized holding loss on available for sale securities		(944)	$ 30
Balance at end of period		114,919
Net Unrealized Gains (Losses) on Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(489)
Change in net unrealized holding loss on available for sale securities		(944)
Balance at end of period		(1,433)
Pension and Post-retirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(667)
Balance at end of period		(667)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(1,156)
Change in net unrealized holding loss on available for sale securities		(944)
Balance at end of period		$ (2,100)

[1] Derived from the audited December 31, 2017 Consolidated Financial Statements.