Loans and allowance for credit losses (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Troubled Debt Restructurings [Member]
Number of contract	62	50
Premodification outstanding recorded investment	$ 57,372	$ 40,395
Postmodification outstanding recorded investment	42,218	37,142
Troubled Debt Restructurings That Subsequently Defaulted [Member]
Number of contract	23	18
Recorded investment	13,082	14,862
Commercial Loan [Member] | Troubled Debt Restructurings [Member]
Number of contract	2	1
Premodification outstanding recorded investment	125	69
Postmodification outstanding recorded investment	114	69
Commercial Loan [Member] | Troubled Debt Restructurings That Subsequently Defaulted [Member]
Number of contract	1	0
Recorded investment	46	0
Consumer Loan [Member] | Troubled Debt Restructurings [Member]
Number of contract	1	0
Premodification outstanding recorded investment	30	0
Postmodification outstanding recorded investment	28	0
Consumer Loan [Member] | Troubled Debt Restructurings That Subsequently Defaulted [Member]
Number of contract	1	0
Recorded investment	28	0
Residential Real Estate [Member] | Troubled Debt Restructurings [Member]
Number of contract	26	20
Premodification outstanding recorded investment	13,530	11,471
Postmodification outstanding recorded investment	12,283	10,359
Residential Real Estate [Member] | Troubled Debt Restructurings That Subsequently Defaulted [Member]
Number of contract	12	10
Recorded investment	8,163	7,353
Commercial Real Estate [Member] | Troubled Debt Restructurings [Member]
Number of contract	19	20
Premodification outstanding recorded investment	21,814	15,874
Postmodification outstanding recorded investment	18,633	14,175
Commercial Real Estate [Member] | Troubled Debt Restructurings That Subsequently Defaulted [Member]
Number of contract	5	5
Recorded investment	3,557	6,751
Construction Loans [Member] | Troubled Debt Restructurings [Member]
Number of contract	14	9
Premodification outstanding recorded investment	21,873	12,981
Postmodification outstanding recorded investment	18,366	12,539
Construction Loans [Member] | Troubled Debt Restructurings That Subsequently Defaulted [Member]
Number of contract	4	3
Recorded investment	$ 1,288	$ 758