Allowance for loan losses (Details Textual) (Details) (USD $)	3 Months Ended			3 Months Ended	12 Months Ended		3 Months Ended
	Mar. 31, 2012 payment_cycles	Dec. 31, 2011	Mar. 31, 2011	Mar. 31, 2012 Acquired Non Impaired Loans [Member]	Dec. 31, 2011 Acquired Non Impaired Loans [Member]	Mar. 31, 2012 Loans with No Related Allowance [Member]	Mar. 31, 2012 Covered Loans [Member]	Mar. 31, 2011 Covered Loans [Member]	Dec. 31, 2011 Covered Loans [Member]	Dec. 31, 2010 Covered Loans [Member]
Financing Receivable, Modifications [Line Items]
Minimum Non Performing Substandard Loans Outstanding for Allowance for Credit Losses	$ 300,000
Minimum Non Performing Substandard Doubtful Loans Outstanding for Allowance for Credit Losses	100,000
Loans and Leases Receivable, Allowance	103,849,000	107,699,000	114,690,000				41,070,000	28,405,000	36,417,000	13,733,000
Provision for Loan Losses Before Benefit Attributable to FDIC Loss Share Agreements							10,831,000	20,510,000
Benefit Attributable to FDIC Loss Share Agreements							(4,899,000)	(15,179,000)
Net provision for covered loan losses				400,000	700,000		5,932,000	5,331,000
Maximum term in which concessionary modifications are considered short-term	90 days
Minimum number of consecutive payment cycles after restructuring date with modified term	6
Impaired Financing Receivable, Recorded Investment	$ 105,156,000	$ 111,610,000	$ 96,242,000			$ 200,000