Loans and allowance for credit losses (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Dec. 31, 2012
Unpaid principal balance	$ 57,927	$ 105,779	$ 57,927	$ 105,779
Recorded investment with no allowance	38,745	77,664	38,745	77,664
Recorded investment with an allowance	9,315	11,163	9,315	11,163
Related allowance	2,167	2,201	2,167	2,201
Average recorded investment	68,071	90,193	75,901	87,064
Commercial Loan [Member]
Unpaid principal balance	1,962	1,677	1,962	1,677
Recorded investment with no allowance	647	715	647	715
Recorded investment with an allowance	305	0	305	0
Related allowance	305	0	305	0
Average recorded investment	853	1,231	753	1,625
Consumer Loan [Member]
Unpaid principal balance	87	92	87	92
Recorded investment with no allowance	19	39	19	39
Recorded investment with an allowance	61	48	61	48
Related allowance	61	48	61	48
Average recorded investment	60	50	52	55
Residential Real Estate [Member]
Unpaid principal balance	24,252	24,992	24,252	24,992
Recorded investment with no allowance	18,429	16,486	18,429	16,486
Recorded investment with an allowance	3,742	2,063	3,742	2,063
Related allowance	924	598	924	598
Average recorded investment	22,131	19,418	20,154	22,536
Commercial Real Estate [Member]
Unpaid principal balance	17,864	37,395	17,864	37,395
Recorded investment with no allowance	11,264	29,718	11,264	29,718
Recorded investment with an allowance	3,655	2,729	3,655	2,729
Related allowance	455	614	455	614
Average recorded investment	20,893	30,987	25,454	29,227
Construction Loans [Member]
Unpaid principal balance	13,762	41,623	13,762	41,623
Recorded investment with no allowance	8,386	30,706	8,386	30,706
Recorded investment with an allowance	1,552	6,323	1,552	6,323
Related allowance	422	941	422	941
Average recorded investment	24,134	38,507	29,488	33,621
Impaired Nonaccrual Loans [Member]
Unpaid principal balance	27,368	53,426	27,368	53,426
Recorded investment with no allowance	13,054	25,311	13,054	25,311
Recorded investment with an allowance	4,447	11,163	4,447	11,163
Related allowance	1,135	2,201	1,135	2,201
Average recorded investment	28,803	38,624	30,119	44,468
Impaired Nonaccrual Loans [Member] | Commercial Loan [Member]
Unpaid principal balance	1,678	1,556	1,678	1,556
Recorded investment with no allowance	546	594	546	594
Recorded investment with an allowance	122	0	122	0
Related allowance	122	0	122	0
Average recorded investment	656	1,126	595	1,538
Impaired Nonaccrual Loans [Member] | Consumer Loan [Member]
Unpaid principal balance	54	92	54	92
Recorded investment with no allowance	19	39	19	39
Recorded investment with an allowance	28	48	28	48
Related allowance	28	48	28	48
Average recorded investment	43	50	44	55
Impaired Nonaccrual Loans [Member] | Residential Real Estate [Member]
Unpaid principal balance	7,486	17,975	7,486	17,975
Recorded investment with no allowance	4,082	9,469	4,082	9,469
Recorded investment with an allowance	1,323	2,063	1,323	2,063
Related allowance	309	598	309	598
Average recorded investment	10,825	13,294	10,921	17,472
Impaired Nonaccrual Loans [Member] | Commercial Real Estate [Member]
Unpaid principal balance	6,287	19,515	6,287	19,515
Recorded investment with no allowance	1,825	11,838	1,825	11,838
Recorded investment with an allowance	1,517	2,729	1,517	2,729
Related allowance	255	614	255	614
Average recorded investment	7,607	13,554	9,905	12,975
Impaired Nonaccrual Loans [Member] | Construction Loans [Member]
Unpaid principal balance	11,863	14,288	11,863	14,288
Recorded investment with no allowance	6,582	3,371	6,582	3,371
Recorded investment with an allowance	1,457	6,323	1,457	6,323
Related allowance	421	941	421	941
Average recorded investment	9,672	10,600	8,654	12,428
Impaired Accruing Restructured Loans [Member]
Unpaid principal balance	29,439	52,353	29,439	52,353
Recorded investment with no allowance	25,691	52,353	25,691	52,353
Recorded investment with an allowance	3,748	0	3,748	0
Related allowance	677	0	677	0
Average recorded investment	38,706	51,569	45,502	42,596
Impaired Accruing Restructured Loans [Member] | Commercial Loan [Member]
Unpaid principal balance	101	121	101	121
Recorded investment with no allowance	101	121	101	121
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	105	105	112	87
Impaired Accruing Restructured Loans [Member] | Consumer Loan [Member]
Unpaid principal balance	0	0	0	0
Recorded investment with no allowance	0	0	0	0
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	0	0	0	0
Impaired Accruing Restructured Loans [Member] | Residential Real Estate [Member]
Unpaid principal balance	16,735	7,017	16,735	7,017
Recorded investment with no allowance	14,347	7,017	14,347	7,017
Recorded investment with an allowance	2,388	0	2,388	0
Related allowance	584	0	584	0
Average recorded investment	11,290	6,124	9,225	5,064
Impaired Accruing Restructured Loans [Member] | Commercial Real Estate [Member]
Unpaid principal balance	10,704	17,880	10,704	17,880
Recorded investment with no allowance	9,439	17,880	9,439	17,880
Recorded investment with an allowance	1,265	0	1,265	0
Related allowance	92	0	92	0
Average recorded investment	12,849	17,433	15,331	16,252
Impaired Accruing Restructured Loans [Member] | Construction Loans [Member]
Unpaid principal balance	1,899	27,335	1,899	27,335
Recorded investment with no allowance	1,804	27,335	1,804	27,335
Recorded investment with an allowance	95	0	95	0
Related allowance	1	0	1	0
Average recorded investment	14,462	27,907	20,834	21,193
Impired Adversely Rated Loans [Member]
Unpaid principal balance	1,120	0	1,120	0
Recorded investment with no allowance	0	0	0	0
Recorded investment with an allowance	1,120	0	1,120	0
Related allowance	355	0	355	0
Average recorded investment	562	0	280	0
Impired Adversely Rated Loans [Member] | Commercial Loan [Member]
Unpaid principal balance	183	0	183	0
Recorded investment with no allowance	0	0	0	0
Recorded investment with an allowance	183	0	183	0
Related allowance	183	0	183	0
Average recorded investment	92	0	46	0
Impired Adversely Rated Loans [Member] | Consumer Loan [Member]
Unpaid principal balance	33	0	33	0
Recorded investment with no allowance	0	0	0	0
Recorded investment with an allowance	33	0	33	0
Related allowance	33	0	33	0
Average recorded investment	17	0	8	0
Impired Adversely Rated Loans [Member] | Residential Real Estate [Member]
Unpaid principal balance	31	0	31	0
Recorded investment with no allowance	0	0	0	0
Recorded investment with an allowance	31	0	31	0
Related allowance	31	0	31	0
Average recorded investment	16	0	8	0
Impired Adversely Rated Loans [Member] | Commercial Real Estate [Member]
Unpaid principal balance	873	0	873	0
Recorded investment with no allowance	0	0	0	0
Recorded investment with an allowance	873	0	873	0
Related allowance	108	0	108	0
Average recorded investment	437	0	218	0
Impired Adversely Rated Loans [Member] | Construction Loans [Member]
Unpaid principal balance	0	0	0	0
Recorded investment with no allowance	0	0	0	0
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	$ 0	$ 0	$ 0	$ 0