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, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Derivative [Line Items]
Interest rate contracts	$ (19)	$ 24
Interest Expense - Subordinated Debentures [Member]
Derivative [Line Items]
Interest expense, Fair Value	(96)	(97)
Other Income [Member]
Derivative [Line Items]
Other income	0	0
Accrued Interest and Other Liabilities [Member]
Derivative [Line Items]
Interest rate contracts	$ (974)		$ (946)