Premises and Equipment - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 Y Bank Office	Dec. 31, 2011	Dec. 31, 2010
Property, Plant and Equipment [Line Items]
Estimated cost of construction projects in progress	$ 1,000,000	$ 1,000,000
Depreciation expense	5,032,000	4,379,000	3,330,000
Lease initial term	10
Number of bank offices	12
Number of mortgage offices	7
Rental Expenses	$ 1,708,000	$ 1,697,000	$ 880,000
Non Cancellable Operating Leases [Member]
Property, Plant and Equipment [Line Items]
Lease initial term	3