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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Derivative [Line Items]
Interest rate contracts	$ (31)	$ 63	$ 5	$ 45
Accrued Interest and Other Liabilities [Member]
Derivative [Line Items]
Interest rate contract, fair value	(728)		(728)		$ (736)
Interest Expense - Subordinated Debentures [Member]
Derivative [Line Items]
Interest expense, Fair Value	(86)	(94)	(176)	(189)
Other Income [Member]
Derivative [Line Items]
Other income	$ 0	$ 0	$ 0	$ 0