Loans and allowance for credit losses (Details 3) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Loan Risk Rating	$ 809,958	$ 841,050	$ 877,331
Pass [Member]
Loan Risk Rating	623,607	643,258
Special Mention [Member]
Loan Risk Rating	67,142	63,686
Substandard [Member]
Loan Risk Rating	70,754	82,214
Doubtful [Member]
Loan Risk Rating	497	522
Non Accrual [Member]
Loan Risk Rating	47,958	51,370
Commercial Loan [Member]
Loan Risk Rating	63,004	69,485	74,485
Commercial Loan [Member] | Pass [Member]
Loan Risk Rating	56,049	59,178
Commercial Loan [Member] | Special Mention [Member]
Loan Risk Rating	3,669	3,813
Commercial Loan [Member] | Substandard [Member]
Loan Risk Rating	1,616	4,748
Commercial Loan [Member] | Doubtful [Member]
Loan Risk Rating	61	77
Commercial Loan [Member] | Non Accrual [Member]
Loan Risk Rating	1,609	1,669
Consumer Loan [Member]
Loan Risk Rating	14,150	14,639	15,265
Consumer Loan [Member] | Pass [Member]
Loan Risk Rating	13,846	14,520
Consumer Loan [Member] | Special Mention [Member]
Loan Risk Rating	185	32
Consumer Loan [Member] | Substandard [Member]
Loan Risk Rating	64	59
Consumer Loan [Member] | Doubtful [Member]
Loan Risk Rating	0	0
Consumer Loan [Member] | Non Accrual [Member]
Loan Risk Rating	55	28
Residential Real Estate [Member]
Loan Risk Rating	306,093	321,604	332,134
Residential Real Estate [Member] | Pass [Member]
Loan Risk Rating	249,611	261,910
Residential Real Estate [Member] | Special Mention [Member]
Loan Risk Rating	17,208	13,467
Residential Real Estate [Member] | Substandard [Member]
Loan Risk Rating	18,438	25,676
Residential Real Estate [Member] | Doubtful [Member]
Loan Risk Rating	436	445
Residential Real Estate [Member] | Non Accrual [Member]
Loan Risk Rating	20,400	20,106
Commercial Real Estate [Member]
Loan Risk Rating	315,644	315,439	327,307
Commercial Real Estate [Member] | Pass [Member]
Loan Risk Rating	257,072	257,247
Commercial Real Estate [Member] | Special Mention [Member]
Loan Risk Rating	19,447	16,001
Commercial Real Estate [Member] | Substandard [Member]
Loan Risk Rating	27,489	28,179
Commercial Real Estate [Member] | Doubtful [Member]
Loan Risk Rating	0	0
Commercial Real Estate [Member] | Non Accrual [Member]
Loan Risk Rating	11,636	14,012
Construction Loans [Member]
Loan Risk Rating	111,067	119,883	128,140
Construction Loans [Member] | Pass [Member]
Loan Risk Rating	47,029	50,403
Construction Loans [Member] | Special Mention [Member]
Loan Risk Rating	26,633	30,373
Construction Loans [Member] | Substandard [Member]
Loan Risk Rating	23,147	23,552
Construction Loans [Member] | Doubtful [Member]
Loan Risk Rating	0	0
Construction Loans [Member] | Non Accrual [Member]
Loan Risk Rating	$ 14,258	$ 15,555