Derivative Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Total notional amount of the Bank's swaps	$ 238.2	$ 240.1
Interest-Rate Swap Agreements With Customers [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Number of interest-rate swap agreements	88
Interest-Rate Swap Agreements With Counterparty Bank [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Number of interest-rate swap agreements	88