Fair Value Measurement - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011
Fair Value Disclosures [Abstract]
Transfer between level 1 and level 2 due to in the valuation of interest rate caps are back-tested			$ 0	$ 0
Transfer between level 2 and level 3 due to reconciliation of all assets measured at fair value on a recurring basis			0	0
Fair value of the collateral dependent loans, net carrying amount	2,200,000	57,100,000	2,200,000	57,100,000	9,900,000
Specific allowance for collateral dependent loans	808,000	2,100,000	808,000	2,100,000	3,000,000
Increase (Decrease) in provision for loan losses	307,000	2,300,000	2,600,000	794,000	27,000
Discount applied to appraisals for estimated selling costs percentage			10.00%
Mortgage servicing rights, fair amount	387,000	4,200,000	387,000	4,200,000	4,900,000
Mortgage servicing rights outstanding	391,000	6,200,000	391,000	6,200,000	5,600,000
Valuation allowance	4,000	2,016,000	4,000	2,016,000	680,000	1,785,000
Mortgage servicing rights net recovery/charge					1,100,000
Other real estate owned carried at fair value	6,400,000	18,500,000	6,400,000	18,500,000	10,100,000
Valuation allowance related to other real estate owned	6,700,000	6,900,000	6,700,000	6,900,000	6,800,000
Additional expenses related to other real estate owned	$ 464,000	$ 1,300,000	$ 1,800,000	$ 2,000,000	$ 2,200,000
Maximum maturity period of short term borrowings					90 days