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, 2019	Mar. 31, 2018	Dec. 31, 2018
Derivative [Line Items]
Interest expense – subordinated debentures	$ (95)	$ 62
Accrued interest and other liabilities
Derivative [Line Items]
Interest rate contracts	(321)		$ (201)
Interest expense – subordinated debentures
Derivative [Line Items]
Interest expense, fair value	(6)	(58)
Other income
Derivative [Line Items]
Other income	$ 0	$ 0