Changes in Accumulated Other Comprehensive Income (Loss) - Components of Accumulated Other Comprehensive Income (Loss) Net of Taxes (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 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		$ (377)	$ 39	(1,536)	$ 359
Balance at end of period		116,547		116,547
Net Unrealized Losses on Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(1,648)		(489)
Change in net unrealized holding loss on available-for-sale securities		(377)		(1,536)
Balance at end of period		(2,025)		(2,025)
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 Loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(2,315)		(1,156)
Change in net unrealized holding loss on available-for-sale securities		(377)		(1,536)
Balance at end of period		$ (2,692)		$ (2,692)

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