Allowance for loan losses (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	$ 108,378	$ 113,090	$ 111,610
Unpaid Principal Balance	134,017	138,761	140,563
Related Allowance	8,444	6,671	7,080
Average Recorded Investment	124,814	124,060	120,838
Total impaired loans with no related allowance [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	36,281	38,326	48,331
Unpaid Principal Balance	55,255	54,809	75,286
Related Allowance	0	0	0
Average Recorded Investment	50,871	46,300	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	1,443	843	6,999
Unpaid Principal Balance	10,154	1,831	9,121
Related Allowance	0	0	0
Average Recorded Investment	9,903	1,613	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	16,910	28,728	29,566
Unpaid Principal Balance	22,337	40,092	46,744
Related Allowance	0	0	0
Average Recorded Investment	19,217	34,838	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	3,370	5,883	7,522
Unpaid Principal Balance	3,944	8,657	13,675
Related Allowance	0	0	0
Average Recorded Investment	4,080	6,850	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	2,902	0	0
Unpaid Principal Balance	4,739	0	0
Related Allowance	0	0	0
Average Recorded Investment	4,888	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	959	0	0
Unpaid Principal Balance	1,239	0	0
Related Allowance	0	0	0
Average Recorded Investment	1,237	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	4	0	0
Unpaid Principal Balance	4	0	0
Related Allowance	0	0	0
Average Recorded Investment	4	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	10,693	2,872	4,244
Unpaid Principal Balance	12,838	4,229	5,746
Related Allowance	0	0	0
Average Recorded Investment	11,542	2,999	4,450
Total impaired loans with a related allowance [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment	72,097	74,764	63,279
Unpaid Principal Balance	78,762	83,952	65,277
Related Allowance	8,444	6,671	7,080
Average Recorded Investment	73,943	77,760	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	8,551	3,428	1,270
Unpaid Principal Balance	12,555	4,725	1,769
Related Allowance	2,900	256	2,497
Average Recorded Investment	10,163	4,053	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	11,543	10,560	6,521
Unpaid Principal Balance	14,113	15,444	6,789
Related Allowance	1,953	2,064	1,599
Average Recorded Investment	10,559	12,286	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	726	5,906	1,798
Unpaid Principal Balance	726	8,840	2,864
Related Allowance	117	1,264	99
Average Recorded Investment	733	6,386	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	29,675	34,654	33,571
Unpaid Principal Balance	29,675	34,703	33,723
Related Allowance	1,674	1,568	1,382
Average Recorded Investment	30,361	34,485	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	5,963	4,413	4,763
Unpaid Principal Balance	5,963	4,412	4,763
Related Allowance	95	52	31
Average Recorded Investment	6,206	4,581	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	1,763	2,359	2,202
Unpaid Principal Balance	1,763	2,359	2,202
Related Allowance	106	121	108
Average Recorded Investment	1,975	2,560	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	13,876	13,444	13,154
Unpaid Principal Balance	13,967	13,469	13,167
Related Allowance	1,599	1,346	1,364
Average Recorded Investment	$ 13,946	$ 13,409	$ 13,155