Loans and Allowance for Loan Losses - Additional Information (Detail) (USD $)	1 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Covered loans acquired in FDIC-assisted transactions	$ 507,700,000	$ 571,500,000
Nonaccrual loans		200,000		200,000
Loan-to-value ratio		110.00%		110.00%
Modified loans	40,300,000	37,200,000	40,300,000	37,200,000
Troubled debt restructurings	23,900,000	26,100,000	23,900,000	26,100,000
Troubled debt restructurings, previous charge-offs			1,900,000	1,700,000
Allowance for loan losses allocated to troubled debt restructurings			640,000	2,700,000
Troubled debt restructurings, provision for loan loss expense			2,600,000	2,400,000	1,700,000
Minimum [Member]
Non-covered loans on nonaccrual status	38,900,000	70,800,000	38,900,000	70,800,000	79,300,000
Troubled Debt Restructuring [Member]
Nonaccrual loans		$ 100,000		$ 100,000