Allowance for loan losses (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	$ 105,156	$ 96,242	$ 111,610
Unpaid Principal Balance	127,770	121,548	138,970
Related Allowance	7,237	7,112	7,080
Average Recorded Investment	107,771	100,005	120,838
Total impaired loans with no related allowance [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	41,402	42,882	48,331
Unpaid Principal Balance	63,427	56,197	73,802
Related Allowance	0	0	0
Average Recorded Investment	43,927	44,658	60,520
Total impaired loans with no related allowance [Member] | Commercial Loan [Member] | Commercial and Industrial Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	3,272	3,372	6,999
Unpaid Principal Balance	9,937	5,498	9,121
Related Allowance	0	0	0
Average Recorded Investment	4,357	3,801	8,442
Total impaired loans with no related allowance [Member] | Commercial Loan [Member] | Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	25,734	31,175	29,566
Unpaid Principal Balance	36,073	38,066	46,744
Related Allowance	0	0	0
Average Recorded Investment	26,449	32,332	37,720
Total impaired loans with no related allowance [Member] | Commercial Loan [Member] | Commercial Real Estate Construction Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	7,764	5,510	7,522
Unpaid Principal Balance	12,785	8,538	13,675
Related Allowance	0	0	0
Average Recorded Investment	7,945	5,645	9,908
Total impaired loans with no related allowance [Member] | Consumer Loan [Member] | Consumer Installment Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	0	0	0
Unpaid Principal Balance	0	0	0
Related Allowance	0	0	0
Average Recorded Investment	0	0	0
Total impaired loans with no related allowance [Member] | Consumer Loan [Member] | Consumer Home Equity Lines Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	0	0	0
Unpaid Principal Balance	0	0	0
Related Allowance	0	0	0
Average Recorded Investment	0	0	0
Total impaired loans with no related allowance [Member] | Consumer Loan [Member] | Consumer Credit Card Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	223	0	0
Unpaid Principal Balance	223	0	0
Related Allowance	0	0	0
Average Recorded Investment	251	0	0
Total impaired loans with no related allowance [Member] | Consumer Loan [Member] | Consumer Residential Mortgages Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	4,409	2,825	4,244
Unpaid Principal Balance	4,409	4,095	4,262
Related Allowance	0	0	0
Average Recorded Investment	4,925	2,880	4,450
Total impaired loans with a related allowance [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	63,754	53,360	63,279
Unpaid Principal Balance	64,343	65,351	65,168
Related Allowance	7,237	7,112	7,080
Average Recorded Investment	63,844	55,347	60,318
Total impaired loans with a related allowance [Member] | Commercial Loan [Member] | Commercial and Industrial Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	2,807	1,257	1,270
Unpaid Principal Balance	3,206	1,843	1,769
Related Allowance	972	156	2,497
Average Recorded Investment	2,919	1,484	752
Total impaired loans with a related allowance [Member] | Commercial Loan [Member] | Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	6,378	22,376	6,521
Unpaid Principal Balance	6,568	30,084	6,789
Related Allowance	2,478	3,932	1,599
Average Recorded Investment	6,120	24,151	3,247
Total impaired loans with a related allowance [Member] | Commercial Loan [Member] | Commercial Real Estate Construction Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	1,165	7,045	1,798
Unpaid Principal Balance	1,165	10,704	2,864
Related Allowance	224	1,622	99
Average Recorded Investment	1,168	7,262	1,929
Total impaired loans with a related allowance [Member] | Consumer Loan [Member] | Consumer Installment Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	32,378	8,333	33,571
Unpaid Principal Balance	32,378	8,342	33,621
Related Allowance	1,901	336	1,382
Average Recorded Investment	32,504	8,016	33,742
Total impaired loans with a related allowance [Member] | Consumer Loan [Member] | Consumer Home Equity Lines Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	5,522	1,307	4,763
Unpaid Principal Balance	5,522	1,307	4,763
Related Allowance	66	8	31
Average Recorded Investment	5,556	1,334	4,996
Total impaired loans with a related allowance [Member] | Consumer Loan [Member] | Consumer Credit Card Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	1,837	2,836	2,202
Unpaid Principal Balance	1,837	2,836	2,202
Related Allowance	92	162	108
Average Recorded Investment	1,885	2,888	2,497
Total impaired loans with a related allowance [Member] | Consumer Loan [Member] | Consumer Residential Mortgages Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	13,667	10,206	13,154
Unpaid Principal Balance	13,667	10,235	13,160
Related Allowance	1,504	896	1,364
Average Recorded Investment	$ 13,692	$ 10,212	$ 13,155