Financial Instruments with Off-balance Sheet Risk and Derivatives - Additional Information (Detail) (Commercial Loan, USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Commercial Loan
Derivative [Line Items]
Interest rate swap notional amount	$ 44.5	$ 54.0
Interest rate swap fair value	$ 1.5	$ 1.5