Loans and allowance for credit losses (Details 1) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2011
Unpaid principal balance	$ 103,034	$ 90,043
Recorded investment with no allowance	82,117	71,847
Recorded investment with an allowance	3,072	4,731
Related allowance	920	1,466
Average recorded investment	79,199	74,007
Commercial Loan [Member]
Unpaid principal balance	2,246	2,188
Recorded investment with no allowance	1,677	1,738
Recorded investment with an allowance	0	0
Related allowance	0	0
Average recorded investment	1,956	2,650
Consumer Loan [Member]
Unpaid principal balance	59	30
Recorded investment with no allowance	55	28
Recorded investment with an allowance	0	0
Related allowance	0	0
Average recorded investment	46	32
Residential Real Estate [Member]
Unpaid principal balance	29,956	26,223
Recorded investment with no allowance	24,596	20,717
Recorded investment with an allowance	812	3,181
Related allowance	420	1,296
Average recorded investment	23,801	21,523
Commercial Real Estate [Member]
Unpaid principal balance	30,475	26,938
Recorded investment with no allowance	24,453	23,578
Recorded investment with an allowance	2,260	0
Related allowance	500	0
Average recorded investment	25,478	22,584
Construction Loans [Member]
Unpaid principal balance	40,298	34,664
Recorded investment with no allowance	31,336	25,786
Recorded investment with an allowance	0	1,550
Related allowance	0	170
Average recorded investment	27,918	27,218
Impaired Nonaccrual Loans [Member]
Unpaid principal balance	65,803	64,835
Recorded investment with no allowance	44,886	46,639
Recorded investment with an allowance	3,072	4,731
Related allowance	920	1,466
Average recorded investment	50,354	49,180
Impaired Nonaccrual Loans [Member] | Commercial Loan [Member]
Unpaid principal balance	2,178	2,119
Recorded investment with no allowance	1,609	1,669
Recorded investment with an allowance	0	0
Related allowance	0	0
Average recorded investment	1,905	2,539
Impaired Nonaccrual Loans [Member] | Consumer Loan [Member]
Unpaid principal balance	59	30
Recorded investment with no allowance	55	28
Recorded investment with an allowance	0	0
Related allowance	0	0
Average recorded investment	46	32
Impaired Nonaccrual Loans [Member] | Residential Real Estate [Member]
Unpaid principal balance	24,948	22,431
Recorded investment with no allowance	19,588	16,925
Recorded investment with an allowance	812	3,181
Related allowance	420	1,296
Average recorded investment	20,221	15,430
Impaired Nonaccrual Loans [Member] | Commercial Real Estate [Member]
Unpaid principal balance	15,398	17,372
Recorded investment with no allowance	9,376	14,012
Recorded investment with an allowance	2,260	0
Related allowance	500	0
Average recorded investment	12,968	14,624
Impaired Nonaccrual Loans [Member] | Construction Loans [Member]
Unpaid principal balance	23,220	22,883
Recorded investment with no allowance	14,258	14,005
Recorded investment with an allowance	0	1,550
Related allowance	0	170
Average recorded investment	15,214	16,555
Impaired Accruing Restructured Loans [Member]
Unpaid principal balance	37,231	25,208
Recorded investment with no allowance	37,231	25,208
Recorded investment with an allowance	0	0
Related allowance	0	0
Average recorded investment	28,845	24,827
Impaired Accruing Restructured Loans [Member] | Commercial Loan [Member]
Unpaid principal balance	68	69
Recorded investment with no allowance	68	69
Recorded investment with an allowance	0	0
Related allowance	0	0
Average recorded investment	51	111
Impaired Accruing Restructured Loans [Member] | Consumer Loan [Member]
Unpaid principal balance	0	0
Recorded investment with no allowance	0	0
Recorded investment with an allowance	0	0
Related allowance	0	0
Average recorded investment	0	0
Impaired Accruing Restructured Loans [Member] | Residential Real Estate [Member]
Unpaid principal balance	5,008	3,792
Recorded investment with no allowance	5,008	3,792
Recorded investment with an allowance	0	0
Related allowance	0	0
Average recorded investment	3,580	6,093
Impaired Accruing Restructured Loans [Member] | Commercial Real Estate [Member]
Unpaid principal balance	15,077	9,566
Recorded investment with no allowance	15,077	9,566
Recorded investment with an allowance	0	0
Related allowance	0	0
Average recorded investment	12,510	7,960
Impaired Accruing Restructured Loans [Member] | Construction Loans [Member]
Unpaid principal balance	17,078	11,781
Recorded investment with no allowance	17,078	11,781
Recorded investment with an allowance	0	0
Related allowance	0	0
Average recorded investment	$ 12,704	$ 10,663