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, 2016	Mar. 31, 2015	Dec. 31, 2015
Derivative [Line Items]
Interest rate contracts	$ (25)	$ (19)
Accrued Interest and Other Liabilities [Member]
Derivative [Line Items]
Interest rate contract, fair value	(775)		$ (736)
Interest Expense - Subordinated Debentures [Member]
Derivative [Line Items]
Interest expense, Fair Value	(89)	(96)
Other Income [Member]
Derivative [Line Items]
Other income	$ 0	$ 0