Details of Impairment Charges Recognized (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Carrying amount of loans measured at fair value	$ 67,904	$ 69,382
Valuation allowance	(8,628)	(8,531)
Collateral Dependent Loans
Financing Receivable, Impaired [Line Items]
Carrying amount of loans measured at fair value	22,396	23,070
Estimated selling costs considered in carrying amount	2,240	1,839
Valuation allowance	(4,813)	(4,684)
Total fair value	$ 19,823	$ 20,225