Derivative Instruments - 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	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Derivative [Line Items]
Interest rate contracts	$ 62	$ 55
Accrued Interest and Other Liabilities [Member]
Derivative [Line Items]
Interest rate contract, fair value	(83)		$ (161)
Interest Expense - Subordinated Debentures [Member]
Derivative [Line Items]
Interest expense, Fair Value	(58)	(76)
Other Income [Member]
Derivative [Line Items]
Other income	$ 0	$ 0