Financial Instruments with Off-Balance Sheet Risk and Derivatives - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Financial Instrument With Off Balance Sheet Risk And Derivatives [Line Items]
Interest rate swap notional amount	$ 48.5	$ 35.9
Interest rate swap fair value	$ 2.4	$ 1.3