Investments in Debt and Equity Securities Reported Less than Historical Cost (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
Fair value	$ 10,502	$ 6,339
Investments in Debt and Equity Securities, Reported at Less than Historical Cost
Schedule of Investments [Line Items]
Historical cost	16,364	11,925
Fair value	$ 10,502	$ 6,339
Percent of the Corporation's available for sale and held to maturity portfolio	1.10%	0.70%