Allowance for loan losses (Details 4) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012 loans	Dec. 31, 2011 loans	Sep. 30, 2011 loans
Summary of Loans Modified as TDRS
Number of Loans	2,964,000	2,502,000	2,502,000
Financing Receivable, Modifications, Recorded Investment	$ 166,327	$ 150,356	$ 129,979
Unpaid Principal Balance	215,236	185,587	154,309
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	156,000
Recorded Investment	5,256
Noncovered Loans [Member]
Summary of Loans Modified as TDRS
Number of Loans	85,000	64,000	44,000
Financing Receivable, Modifications, Recorded Investment	23,753	15,002	12,607
Unpaid Principal Balance	33,810	20,980	17,036
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	0
Recorded Investment	0
Noncovered Loans [Member] | Commercial and Industrial Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	22,000	14,000	8,000
Financing Receivable, Modifications, Recorded Investment	2,413	2,512	1,096
Unpaid Principal Balance	8,478	4,336	2,654
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	0
Recorded Investment	0
Noncovered Loans [Member] | Commercial Real Estate [Member]
Summary of Loans Modified as TDRS
Number of Loans	36,000	28,000	20,000
Financing Receivable, Modifications, Recorded Investment	17,763	9,167	8,068
Unpaid Principal Balance	21,307	12,659	10,725
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	0
Recorded Investment	0
Noncovered Loans [Member] | Commercial Real Estate Construction Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	27,000	22,000	16,000
Financing Receivable, Modifications, Recorded Investment	3,577	3,323	3,443
Unpaid Principal Balance	4,025	3,985	3,657
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	0
Recorded Investment	0
Noncovered Consumer Loans [Member]
Summary of Loans Modified as TDRS
Number of Loans	2,803,000	2,394,000	2,424,000
Financing Receivable, Modifications, Recorded Investment	65,835	57,934	57,742
Unpaid Principal Balance	70,188	59,601	59,172
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	156,000
Recorded Investment	5,256
Noncovered Consumer Loans [Member] | Consumer Installment [Member]
Summary of Loans Modified as TDRS
Number of Loans	1,840,000	1,547,000	1,568,000
Financing Receivable, Modifications, Recorded Investment	32,577	33,571	34,654
Unpaid Principal Balance	34,414	33,723	34,703
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	122,000
Recorded Investment	4,385
Noncovered Consumer Loans [Member] | Consumer Home Equity Lines Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	229,000	174,000	166,000
Financing Receivable, Modifications, Recorded Investment	6,922	4,763	4,413
Unpaid Principal Balance	7,202	4,763	4,412
Summary of Loans Modified as TDRS Which Defaulted
Recorded Investment	669
Noncovered Consumer Loans [Member] | Consumer Credit Card Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	426,000	496,000	529,000
Financing Receivable, Modifications, Recorded Investment	1,767	2,202	2,359
Unpaid Principal Balance	1,767	2,202	2,359
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	18,000
Recorded Investment	122
Noncovered Consumer Loans [Member] | Consumer Residential Mortgages Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	308,000	177,000	161,000
Financing Receivable, Modifications, Recorded Investment	24,569	17,398	16,316
Unpaid Principal Balance	26,805	18,913	17,698
Summary of Loans Modified as TDRS Which Defaulted
Recorded Investment	80
Consumer Home Equity Lines Financing Receivable [Member]
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	15,000
Covered Loans [Member]
Summary of Loans Modified as TDRS
Number of Loans	46,000	44,000	34,000
Financing Receivable, Modifications, Recorded Investment	71,997	77,420	59,630
Unpaid Principal Balance	106,464	105,006	78,101
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	0
Recorded Investment	0
Covered Loans [Member] | Commercial and Industrial Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	11,000	15,000	8,000
Financing Receivable, Modifications, Recorded Investment	5,278	7,578	7,287
Unpaid Principal Balance	6,216	10,812	10,901
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	0
Recorded Investment	0
Covered Loans [Member] | Commercial Real Estate [Member]
Summary of Loans Modified as TDRS
Number of Loans	25,000	21,000	15,000
Financing Receivable, Modifications, Recorded Investment	52,079	57,786	41,403
Unpaid Principal Balance	57,681	62,159	45,282
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	0
Recorded Investment	0
Covered Loans [Member] | Commercial Real Estate Construction Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	10,000	8,000	11,000
Financing Receivable, Modifications, Recorded Investment	14,640	12,056	10,940
Unpaid Principal Balance	42,567	32,035	21,918
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	0
Recorded Investment	0
Covered Loans [Member] | Consumer Home Equity Lines Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	30,000
Financing Receivable, Modifications, Recorded Investment	4,742
Unpaid Principal Balance	$ 4,774
Consumer Residential Mortgages Financing Receivable [Member]
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	1,000