Loans - (Narrative) (Detail) (USD $)	1 Months Ended			6 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Sep. 30, 2011	Sep. 30, 2012	Dec. 31, 2011
Covered loans acquired in FDIC-assisted transactions	$ 546,200,000	$ 571,500,000	$ 595,400,000
Nonaccrual loans	200,000				200,000
Loan-to-value ratio	110.00%				110.00%
Modified loans	23,500,000		27,000,000	27,000,000	23,500,000
Troubled debt restructurings	23,300,000	26,100,000			23,300,000	26,100,000
Troubled debt restructurings, previous charge-offs					2,100,000	1,700,000
Allowance for loan losses allocated to troubled debt restructurings					676,000.0	2.7
Troubled debt restructurings, provision for loan loss expense				1,600,000	2,300,000	2,400,000
Troubled Debt Restructuring [Member]
Nonaccrual loans	$ 100,000				$ 100,000