Loans and allowance for credit losses (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Impaired Nonaccrual Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	$ 26,702	$ 21,554
Recorded investment with no allowance	11,037	12,422
Recorded investment with an allowance	8,119	2,204
Related allowance	2,049	716
Year-to-date average recorded investment	20,224	26,006
Impaired Nonaccrual Loans [Member] | Construction Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	12,805	6,787
Recorded investment with no allowance	4,383	3,709
Recorded investment with an allowance	3,828	240
Related allowance	340	203
Year-to-date average recorded investment	7,969	7,270
Impaired Nonaccrual Loans [Member] | Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	5,064	7,692
Recorded investment with no allowance	3,363	3,862
Recorded investment with an allowance	382	1,304
Related allowance	115	225
Year-to-date average recorded investment	6,393	10,240
Impaired Nonaccrual Loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	6,792	5,218
Recorded investment with no allowance	2,566	4,261
Recorded investment with an allowance	3,811	410
Related allowance	1,504	38
Year-to-date average recorded investment	5,162	7,829
Impaired Nonaccrual Loans [Member] | Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	1,991	1,801
Recorded investment with no allowance	710	547
Recorded investment with an allowance	71	245
Related allowance	63	245
Year-to-date average recorded investment	686	619
Impaired Nonaccrual Loans [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	50	56
Recorded investment with no allowance	15	43
Recorded investment with an allowance	27	5
Related allowance	27	5
Year-to-date average recorded investment	14	48
Impaired Accruing TDRs [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	25,333	26,088
Recorded investment with no allowance	21,918	23,805
Recorded investment with an allowance	3,415	2,283
Related allowance	159	67
Year-to-date average recorded investment	25,711	38,919
Impaired Accruing TDRs [Member] | Construction Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	1,306	1,620
Recorded investment with no allowance	1,214	1,527
Recorded investment with an allowance	92	93
Related allowance	1	1
Year-to-date average recorded investment	1,463	14,405
Impaired Accruing TDRs [Member] | Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	16,046	14,582
Recorded investment with no allowance	13,555	13,177
Recorded investment with an allowance	2,491	1,405
Related allowance	143	60
Year-to-date average recorded investment	15,314	11,101
Impaired Accruing TDRs [Member] | Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	7,919	9,791
Recorded investment with no allowance	7,140	9,006
Recorded investment with an allowance	779	785
Related allowance	6	6
Year-to-date average recorded investment	8,855	13,308
Impaired Accruing TDRs [Member] | Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	62	95
Recorded investment with no allowance	9	95
Recorded investment with an allowance	53	0
Related allowance	9	0
Year-to-date average recorded investment	79	105
Impaired Accruing TDRs [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	0	0
Recorded investment with no allowance	0	0
Recorded investment with an allowance	0	0
Related allowance	0	0
Year-to-date average recorded investment	0	0
Total Impired Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	52,035	47,642
Recorded investment with no allowance	32,955	36,227
Recorded investment with an allowance	11,534	4,487
Related allowance	2,208	783
Year-to-date average recorded investment	45,935	64,925
Total Impired Loans [Member] | Construction Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	14,111	8,407
Recorded investment with no allowance	5,597	5,236
Recorded investment with an allowance	3,920	333
Related allowance	341	204
Year-to-date average recorded investment	9,432	21,675
Total Impired Loans [Member] | Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	21,110	22,274
Recorded investment with no allowance	16,918	17,039
Recorded investment with an allowance	2,873	2,709
Related allowance	258	285
Year-to-date average recorded investment	21,707	21,341
Total Impired Loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	14,711	15,009
Recorded investment with no allowance	9,706	13,267
Recorded investment with an allowance	4,590	1,195
Related allowance	1,510	44
Year-to-date average recorded investment	14,017	21,137
Total Impired Loans [Member] | Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	2,053	1,896
Recorded investment with no allowance	719	642
Recorded investment with an allowance	124	245
Related allowance	72	245
Year-to-date average recorded investment	765	724
Total Impired Loans [Member] | Consumer Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	50	56
Recorded investment with no allowance	15	43
Recorded investment with an allowance	27	5
Related allowance	27	5
Year-to-date average recorded investment	$ 14	$ 48