Fair Value - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities held by the company	$ 612,396		$ 612,396		$ 516,844
Loan held for sale, at fair value	18,993		18,993		21,485
Contractual principal balance of Loan held for sale	18,784		18,784		21,225
Difference in amount of loan held for sale	209		209		260
Impaired loans, carrying amount	9,786		9,786		9,729
Valuation allowance for loans losses	5,530		5,530		4,911
Provision for Loan Losses	393		1,023		4,226
Other real estate, carrying value	150		150		250
Loss on sales/write-downs of other real estate	100	230	100	230
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities held by the company	12,200		12,200
Significant Unobservable Inputs (Level 3) [Member] | Obligations of State and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities held by the company	11,800		11,800
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities held by the company	$ 353		$ 353
Maximum [Member] | Short-term Borrowings [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Maturity duration of short-term borrowing			90 days