Accumulated Other Comprehensive Loss, net - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ 114,554,000	$ 41,705,000
Balance at end of period	117,476,000	114,554,000
Net Unrealized Gains (Losses) on Available-for-sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(489,000)	(520,000)
Change in net unrealized holding losses on available-for-sale securities, before reclassification, net of tax benefit	(763,000)	114,000
Reclassification for previously unrealized net gains recognized in income, net of tax expense	0	(2,000)
Net current period other comprehensive gain (loss)		112,000
Tax Cuts and Jobs Act of 2017, reclassification from AOCI to retained earnings		(81,000)
Balance at end of period	(1,252,000)	(489,000)
Pension and Post-retirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(667,000)	(715,000)
Reclassification for previously unrealized net gains recognized in income, net of tax expense	0
Net gain on pension and post-retirement benefit plans, net of tax expense	592,000	155,000
Net current period other comprehensive gain (loss)		155,000
Tax Cuts and Jobs Act of 2017, reclassification from AOCI to retained earnings		(107,000)
Balance at end of period	(75,000)	(667,000)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(1,156,000)	(1,235,000)
Change in net unrealized holding losses on available-for-sale securities, before reclassification, net of tax benefit	(763,000)	114,000
Reclassification for previously unrealized net gains recognized in income, net of tax expense		(2,000)
Net gain on pension and post-retirement benefit plans, net of tax expense	592,000	155,000
Net current period other comprehensive gain (loss)		267,000
Tax Cuts and Jobs Act of 2017, reclassification from AOCI to retained earnings		(188,000)
Balance at end of period	$ (1,327,000)	$ (1,156,000)