Interest Rate Swaps - Amounts and Locations of Activity Related to Interest Rate Swaps Designated as Cash Flow Hedges within Corporation's Consolidated Balance Sheet and Statement of Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Interest rate contracts	$ 194	$ 180	$ 137
Accrued Interest Payable and Other Liabilities [Member]
Derivative [Line Items]
Interest rate contract, fair value	(161)	(459)
Interest Expense - Subordinated Debentures [Member]
Derivative [Line Items]
Interest expense, Fair Value	(288)	(340)	(378)
Other Income [Member]
Derivative [Line Items]
Other income	$ 0	$ 0	$ 0