Loans and reserve for credit losses (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
Reserve for loan losses Total	$ 22,694	$ 24,548	$ 27,261	$ 38,219	$ 43,951	$ 43,905
Recorded investment in loans Total	912,574		858,164
Commerical real estate [Member]
Reserve for loan losses Individually evaluated for impairment	2,152		1,088
Reserve for loan losses Collectively evaluated for impairment	8,560		10,508
Reserve for loan losses Total	10,712	11,225	11,596	18,103	22,314	21,648
Recorded investment in loans Individually evaluated for impairment	42,477		42,859
Recorded investment in loans, Collectively evaluated for impairment	491,400		482,442
Recorded investment in loans Total	533,877		525,301
Construction [Member]
Reserve for loan losses Individually evaluated for impairment	5		440
Reserve for loan losses Collectively evaluated for impairment	837		1,143
Reserve for loan losses Total	842	1,236	1,583	4,529	5,060	5,398
Recorded investment in loans Individually evaluated for impairment	1,664		9,734
Recorded investment in loans, Collectively evaluated for impairment	46,541		35,916
Recorded investment in loans Total	48,205		45,650
Residential real estate [Member]
Reserve for loan losses Individually evaluated for impairment	173		1,141
Reserve for loan losses Collectively evaluated for impairment	2,799		2,410
Reserve for loan losses Total	2,972	3,714	3,551	3,693	3,864	3,259
Recorded investment in loans Individually evaluated for impairment	669		4,840
Recorded investment in loans, Collectively evaluated for impairment	95,106		80,654
Recorded investment in loans Total	95,775		85,494
Commercial and Industrial [Member]
Reserve for loan losses Individually evaluated for impairment	331		829
Reserve for loan losses Collectively evaluated for impairment	5,017		6,438
Reserve for loan losses Total	5,348	5,676	7,267	8,785	9,908	11,291
Recorded investment in loans Individually evaluated for impairment	6,922		9,602
Recorded investment in loans, Collectively evaluated for impairment	190,055		152,611
Recorded investment in loans Total	196,977		162,213
Consumer [Member]
Reserve for loan losses Individually evaluated for impairment	0		301
Reserve for loan losses Collectively evaluated for impairment	1,690		1,876
Reserve for loan losses Total	1,690	2,039	2,177	2,508	2,788	2,292
Recorded investment in loans Individually evaluated for impairment	0		1,636
Recorded investment in loans, Collectively evaluated for impairment	37,740		37,870
Recorded investment in loans Total	37,740		39,506
Allocated Financing Receivables [Member]
Reserve for loan losses Individually evaluated for impairment	2,661		3,799
Reserve for loan losses Collectively evaluated for impairment	18,903		22,375
Reserve for loan losses Total	21,564		26,174
Recorded investment in loans Individually evaluated for impairment	51,732		68,671
Recorded investment in loans, Collectively evaluated for impairment	860,842		789,493
Recorded investment in loans Total	912,574		858,164
Unallocated [Member]
Reserve for loan losses Total	$ 1,130	$ 658	$ 1,087	$ 601	$ 17	$ 17