Changes in Accumulated Other Comprehensive Income (Loss) - Components of Accumulated Other Comprehensive Income (Loss) Net of Taxes (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]
Balance at beginning of period	[1]			$ 114,554
Change in net unrealized holding loss on available for sale securities		$ (215)	$ 290	(1,159)	$ 320
Balance at end of period		115,724		115,724
Net Unrealized Gains (Losses) on Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(1,433)		(489)
Change in net unrealized holding loss on available for sale securities		(215)		(1,159)
Balance at end of period		(1,648)		(1,648)
Pension and Post-retirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(667)		(667)
Balance at end of period		(667)		(667)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(2,100)		(1,156)
Change in net unrealized holding loss on available for sale securities		(215)		(1,159)
Balance at end of period		$ (2,315)		$ (2,315)

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