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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Derivative [Line Items]
Interest expense – subordinated debentures	$ (162)	$ 37	$ (257)	$ 99
Accrued interest and other liabilities
Derivative [Line Items]
Interest rate contracts	(526)		(526)		$ (201)
Interest expense – subordinated debentures
Derivative [Line Items]
Interest expense, fair value	(14)	(46)	(20)	(105)
Other income
Derivative [Line Items]
Other income	$ 0	$ 0	$ 0	$ 0