Allowance for loan losses (Details 4) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012 loans	Dec. 31, 2011 loans
Summary of Loans Modified as TDRS
Number of Loans	2,444,000	2,502,000
Recorded Investment	$ 151,326	$ 150,356
Unpaid Principal Balance	190,805	183,994
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	168,000
Recorded Investment	4,701
Noncovered Loans [Member]
Summary of Loans Modified as TDRS
Number of Loans	72,000	64,000
Recorded Investment	17,117	15,002
Unpaid Principal Balance	24,026	20,980
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	2,000
Recorded Investment	1,288
Noncovered Loans [Member] | Commercial and Industrial Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	15,000	14,000
Recorded Investment	3,388	2,512
Unpaid Principal Balance	6,031	4,336
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	1,000
Recorded Investment	1,169
Noncovered Loans [Member] | Commercial Real Estate [Member]
Summary of Loans Modified as TDRS
Number of Loans	30,000	28,000
Recorded Investment	9,136	9,167
Unpaid Principal Balance	12,745	12,659
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	1,000
Recorded Investment	119
Noncovered Loans [Member] | Commercial Real Estate Construction Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	27,000	22,000
Recorded Investment	4,593	3,323
Unpaid Principal Balance	5,250	3,985
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	2,328,000	2,394,000
Recorded Investment	58,036	57,934
Unpaid Principal Balance	58,036	58,008
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	165,000
Recorded Investment	3,112
Noncovered Consumer Loans [Member] | Consumer Installment [Member]
Summary of Loans Modified as TDRS
Number of Loans	1,506,000	1,547,000
Recorded Investment	32,378	33,571
Unpaid Principal Balance	32,378	33,621
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	122,000
Recorded Investment	2,798
Noncovered Consumer Loans [Member] | Consumer Home Equity Lines Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	184,000	174,000
Recorded Investment	5,522	4,763
Unpaid Principal Balance	5,522	4,763
Noncovered Consumer Loans [Member] | Consumer Credit Card Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	454,000	496,000
Recorded Investment	2,060	2,202
Unpaid Principal Balance	2,060	2,202
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	43,000
Recorded Investment	314
Noncovered Consumer Loans [Member] | Consumer Residential Mortgages Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	184,000	177,000
Recorded Investment	18,076	17,398
Unpaid Principal Balance	18,076	17,422
Covered Loans [Member]
Summary of Loans Modified as TDRS
Number of Loans	44,000	44,000
Recorded Investment	76,173	77,420
Unpaid Principal Balance	108,743	105,006
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	1,000
Recorded Investment	301
Covered Loans [Member] | Commercial and Industrial Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	12,000	15,000
Recorded Investment	7,662	7,578
Unpaid Principal Balance	14,877	10,812
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	23,000	21,000
Recorded Investment	54,824	57,786
Unpaid Principal Balance	60,075	62,159
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	1,000
Recorded Investment	301
Covered Loans [Member] | Commercial Real Estate Construction Financing Receivable [Member]
Summary of Loans Modified as TDRS
Number of Loans	9,000	8,000
Recorded Investment	13,687	12,056
Unpaid Principal Balance	33,791	32,035
Summary of Loans Modified as TDRS Which Defaulted
Number of Loans	0
Recorded Investment	$ 0