Loans and allowance for credit losses (Details 5) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	$ 709,659	$ 711,919
Construction Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	68,348	64,591
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	277,184	274,857
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	297,762	304,605
Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	56,492	57,195
Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	9,873	10,671
Pass [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	597,125	592,085
Pass [Member] | Construction Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	37,020	39,268
Pass [Member] | Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	242,869	235,054
Pass [Member] | Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	255,486	255,280
Pass [Member] | Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	52,063	52,032
Pass [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	9,687	10,451
Special Mention [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	73,377	72,366
Special Mention [Member] | Construction Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	17,573	15,884
Special Mention [Member] | Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	23,320	22,638
Special Mention [Member] | Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	29,555	30,105
Special Mention [Member] | Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	2,768	3,691
Special Mention [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	161	48
Substandard [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	39,067	46,907
Substandard [Member] | Construction Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	13,755	9,439
Substandard [Member] | Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	10,995	17,114
Substandard [Member] | Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	12,721	19,210
Substandard [Member] | Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	1,571	972
Substandard [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	25	172
Doubtful [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	90	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	90	500
Doubtful [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan Risk Rating	$ 0	$ 0