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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Derivative [Line Items]
Interest rate contracts	$ (19)	$ 99	$ 25	$ (117)
Interest Expense - Subordinated Debentures [Member]
Derivative [Line Items]
Interest expense, Fair Value	(95)	(96)	(284)	(287)
Other Income [Member]
Derivative [Line Items]
Other income	0	$ 0	0	$ 0
Accrued Interest and Other Liabilities [Member]
Derivative [Line Items]
Interest rate contracts	$ (908)		$ (908)		$ (946)