Loans and allowance for credit losses (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013	Dec. 31, 2012
Unpaid principal balance	$ 99,674	$ 105,779	$ 99,674	$ 105,779
Recorded investment with no allowance	65,715	77,664	65,715	77,664
Recorded investment with an allowance	19,381	11,163	19,381	11,163
Related allowance	3,312	2,201	3,312	2,201
Average recorded investment	80,721	90,193	81,777	87,064
Commercial Loan [Member]
Unpaid principal balance	1,720	1,677	1,720	1,677
Recorded investment with no allowance	682	715	682	715
Recorded investment with an allowance	29	0	29	0
Related allowance	29	0	29	0
Average recorded investment	643	1,231	652	1,625
Consumer Loan [Member]
Unpaid principal balance	58	92	58	92
Recorded investment with no allowance	51	39	51	39
Recorded investment with an allowance	0	48	0	48
Related allowance	0	48	0	48
Average recorded investment	51	50	49	55
Residential Real Estate [Member]
Unpaid principal balance	29,295	24,992	29,295	24,992
Recorded investment with no allowance	19,502	16,486	19,502	16,486
Recorded investment with an allowance	2,936	2,063	2,936	2,063
Related allowance	377	598	377	598
Average recorded investment	19,697	19,418	18,802	22,536
Commercial Real Estate [Member]
Unpaid principal balance	30,497	37,395	30,497	37,395
Recorded investment with no allowance	18,638	29,718	18,638	29,718
Recorded investment with an allowance	8,620	2,729	8,620	2,729
Related allowance	2,189	614	2,189	614
Average recorded investment	26,198	30,987	27,727	29,227
Construction Loans [Member]
Unpaid principal balance	38,104	41,623	38,104	41,623
Recorded investment with no allowance	26,842	30,706	26,842	30,706
Recorded investment with an allowance	7,796	6,323	7,796	6,323
Related allowance	717	941	717	941
Average recorded investment	34,132	38,507	34,547	33,621
Impaired Nonaccrual Loans [Member]
Unpaid principal balance	49,396	53,426	49,396	53,426
Recorded investment with no allowance	27,458	25,311	27,458	25,311
Recorded investment with an allowance	7,360	11,163	7,360	11,163
Related allowance	1,580	2,201	1,580	2,201
Average recorded investment	30,329	38,624	30,731	44,468
Impaired Nonaccrual Loans [Member] | Commercial Loan [Member]
Unpaid principal balance	1,611	1,556	1,611	1,556
Recorded investment with no allowance	573	594	573	594
Recorded investment with an allowance	29	0	29	0
Related allowance	29	0	29	0
Average recorded investment	531	1,126	537	1,538
Impaired Nonaccrual Loans [Member] | Consumer Loan [Member]
Unpaid principal balance	58	92	58	92
Recorded investment with no allowance	51	39	51	39
Recorded investment with an allowance	0	48	0	48
Related allowance	0	48	0	48
Average recorded investment	51	50	49	55
Impaired Nonaccrual Loans [Member] | Residential Real Estate [Member]
Unpaid principal balance	22,890	17,975	22,890	17,975
Recorded investment with no allowance	14,948	9,469	14,948	9,469
Recorded investment with an allowance	1,085	2,063	1,085	2,063
Related allowance	301	598	301	598
Average recorded investment	12,883	13,294	11,920	17,472
Impaired Nonaccrual Loans [Member] | Commercial Real Estate [Member]
Unpaid principal balance	13,876	19,515	13,876	19,515
Recorded investment with no allowance	8,135	11,838	8,135	11,838
Recorded investment with an allowance	2,502	2,729	2,502	2,729
Related allowance	650	614	650	614
Average recorded investment	9,784	13,554	10,824	12,975
Impaired Nonaccrual Loans [Member] | Construction Loans [Member]
Unpaid principal balance	10,961	14,288	10,961	14,288
Recorded investment with no allowance	3,751	3,371	3,751	3,371
Recorded investment with an allowance	3,744	6,323	3,744	6,323
Related allowance	600	941	600	941
Average recorded investment	7,080	10,600	7,401	12,428
Impaired Accruing Restructured Loans [Member]
Unpaid principal balance	50,278	52,353	50,278	52,353
Recorded investment with no allowance	38,257	52,353	38,257	52,353
Recorded investment with an allowance	12,021	0	12,021	0
Related allowance	1,732	0	1,732	0
Average recorded investment	50,392	51,569	51,046	42,596
Impaired Accruing Restructured Loans [Member] | Commercial Loan [Member]
Unpaid principal balance	109	121	109	121
Recorded investment with no allowance	109	121	109	121
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	112	105	115	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	6,405	7,017	6,405	7,017
Recorded investment with no allowance	4,554	7,017	4,554	7,017
Recorded investment with an allowance	1,851	0	1,851	0
Related allowance	76	0	76	0
Average recorded investment	6,814	6,124	6,882	5,064
Impaired Accruing Restructured Loans [Member] | Commercial Real Estate [Member]
Unpaid principal balance	16,621	17,880	16,621	17,880
Recorded investment with no allowance	10,503	17,880	10,503	17,880
Recorded investment with an allowance	6,118	0	6,118	0
Related allowance	1,539	0	1,539	0
Average recorded investment	16,414	17,433	16,903	16,252
Impaired Accruing Restructured Loans [Member] | Construction Loans [Member]
Unpaid principal balance	27,143	27,335	27,143	27,335
Recorded investment with no allowance	23,091	27,335	23,091	27,335
Recorded investment with an allowance	4,052	0	4,052	0
Related allowance	117	0	117	0
Average recorded investment	$ 27,052	$ 27,907	$ 27,146	$ 21,193