Loans and allowance for credit losses (Details 5) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	$ 703,637	$ 711,919
Construction Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	66,812	64,591
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	270,480	274,857
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	302,147	304,605
Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	54,014	57,195
Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	10,184	10,671
Pass [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	585,401	592,085
Pass [Member] | Construction Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	39,893	39,268
Pass [Member] | Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	235,953	235,054
Pass [Member] | Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	250,195	255,280
Pass [Member] | Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	49,265	52,032
Pass [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	10,095	10,451
Special Mention [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	74,629	72,366
Special Mention [Member] | Construction Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	15,871	15,884
Special Mention [Member] | Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	23,263	22,638
Special Mention [Member] | Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	31,651	30,105
Special Mention [Member] | Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	3,797	3,691
Special Mention [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	47	48
Substandard [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	43,017	46,907
Substandard [Member] | Construction Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	11,048	9,439
Substandard [Member] | Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	11,264	17,114
Substandard [Member] | Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	20,301	19,210
Substandard [Member] | Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	362	972
Substandard [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	42	172
Doubtful [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	590	561
Doubtful [Member] | Construction Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	0	0
Doubtful [Member] | Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	0	51
Doubtful [Member] | Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	0	10
Doubtful [Member] | Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	590	500
Doubtful [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	$ 0	$ 0