Loans and allowance for credit losses (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012	Dec. 31, 2011
Unpaid principal balance	$ 105,049	$ 90,043	$ 105,049	$ 90,043
Recorded investment with no allowance	90,208	71,847	90,208	71,847
Recorded investment with an allowance	19	4,731	19	4,731
Related allowance	19	1,466	19	1,466
Average recorded investment	86,945	73,814	82,823	74,007
Commercial Loan [Member]
Unpaid principal balance	2,213	2,188	2,213	2,188
Recorded investment with no allowance	1,646	1,738	1,646	1,738
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	1,651	1,715	1,834	2,650
Consumer Loan [Member]
Unpaid principal balance	58	30	58	30
Recorded investment with no allowance	54	28	54	28
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	55	34	50	32
Residential Real Estate [Member]
Unpaid principal balance	25,655	26,223	25,655	26,223
Recorded investment with no allowance	19,671	20,717	19,671	20,717
Recorded investment with an allowance	0	3,181	0	3,181
Related allowance	0	1,296	0	1,296
Average recorded investment	22,212	21,274	23,178	21,523
Commercial Real Estate [Member]
Unpaid principal balance	33,135	26,938	33,135	26,938
Recorded investment with no allowance	28,902	23,578	28,902	23,578
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	27,478	23,494	26,740	22,584
Construction Loans [Member]
Unpaid principal balance	43,988	34,664	43,988	34,664
Recorded investment with no allowance	39,935	25,786	39,935	25,786
Recorded investment with an allowance	19	1,550	19	1,550
Related allowance	19	170	19	170
Average recorded investment	35,549	27,297	31,021	27,218
Impaired Nonaccrual Loans [Member]
Unpaid principal balance	54,264	64,835	54,264	64,835
Recorded investment with no allowance	39,423	46,639	39,423	46,639
Recorded investment with an allowance	19	4,731	19	4,731
Related allowance	19	1,466	19	1,466
Average recorded investment	42,935	49,744	47,014	49,180
Impaired Nonaccrual Loans [Member] | Commercial Loan [Member]
Unpaid principal balance	2,124	2,119	2,124	2,119
Recorded investment with no allowance	1,557	1,669	1,557	1,669
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	1,572	1,680	1,760	2,539
Impaired Nonaccrual Loans [Member] | Consumer Loan [Member]
Unpaid principal balance	58	30	58	30
Recorded investment with no allowance	54	28	54	28
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	55	34	50	32
Impaired Nonaccrual Loans [Member] | Residential Real Estate [Member]
Unpaid principal balance	20,424	22,431	20,424	22,431
Recorded investment with no allowance	14,440	16,925	14,440	16,925
Recorded investment with an allowance	0	3,181	0	3,181
Related allowance	0	1,296	0	1,296
Average recorded investment	17,092	18,118	18,920	15,430
Impaired Nonaccrual Loans [Member] | Commercial Real Estate [Member]
Unpaid principal balance	16,149	17,372	16,149	17,372
Recorded investment with no allowance	11,916	14,012	11,916	14,012
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	11,446	13,545	12,567	14,624
Impaired Nonaccrual Loans [Member] | Construction Loans [Member]
Unpaid principal balance	15,509	22,883	15,509	22,883
Recorded investment with no allowance	11,456	14,005	11,456	14,005
Recorded investment with an allowance	19	1,550	19	1,550
Related allowance	19	170	19	170
Average recorded investment	12,770	16,367	13,717	16,555
Impaired Accruing Restructured Loans [Member]
Unpaid principal balance	50,785	25,208	50,785	25,208
Recorded investment with no allowance	50,785	25,208	50,785	25,208
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	44,010	24,070	35,809	24,827
Impaired Accruing Restructured Loans [Member] | Commercial Loan [Member]
Unpaid principal balance	89	69	89	69
Recorded investment with no allowance	89	69	89	69
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	79	35	74	111
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	5,231	3,792	5,231	3,792
Recorded investment with no allowance	5,231	3,792	5,231	3,792
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	5,120	3,156	4,258	6,093
Impaired Accruing Restructured Loans [Member] | Commercial Real Estate [Member]
Unpaid principal balance	16,986	9,566	16,986	9,566
Recorded investment with no allowance	16,986	9,566	16,986	9,566
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	16,032	9,949	14,173	7,960
Impaired Accruing Restructured Loans [Member] | Construction Loans [Member]
Unpaid principal balance	28,479	11,781	28,479	11,781
Recorded investment with no allowance	28,479	11,781	28,479	11,781
Recorded investment with an allowance	0	0	0	0
Related allowance	0	0	0	0
Average recorded investment	$ 22,779	$ 10,930	$ 17,304	$ 10,663