Loans and reserve for credit losses (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Reserve for loan losses Total	$ 24,548	$ 27,261	$ 43,951	$ 43,905
Recorded investment in loans Total	877,084	858,164
Commerical real estate [Member]
Reserve for loan losses Individually evaluated for impairment	1,312	1,088
Reserve for loan losses Collectively evaluated for impairment	9,913	10,508
Reserve for loan losses Total	11,225	11,596	22,314	21,648
Recorded investment in loans Individually evaluated for impairment	44,060	42,859
Recorded investment in loans, Collectively evaluated for impairment	500,935	482,442
Recorded investment in loans Total	544,995	525,301
Construction [Member]
Reserve for loan losses Individually evaluated for impairment	416	440
Reserve for loan losses Collectively evaluated for impairment	820	1,143
Reserve for loan losses Total	1,236	1,583	5,060	5,398
Recorded investment in loans Individually evaluated for impairment	3,293	9,734
Recorded investment in loans, Collectively evaluated for impairment	35,541	35,916
Recorded investment in loans Total	38,834	45,650
Residential real estate [Member]
Reserve for loan losses Individually evaluated for impairment	1,141	1,141
Reserve for loan losses Collectively evaluated for impairment	2,573	2,410
Reserve for loan losses Total	3,714	3,551	3,864	3,259
Recorded investment in loans Individually evaluated for impairment	4,833	4,840
Recorded investment in loans, Collectively evaluated for impairment	88,642	80,654
Recorded investment in loans Total	93,475	85,494
Commercial and industrial [Member]
Reserve for loan losses Individually evaluated for impairment	1,230	829
Reserve for loan losses Collectively evaluated for impairment	4,446	6,438
Reserve for loan losses Total	5,676	7,267	9,908	11,291
Recorded investment in loans Individually evaluated for impairment	9,165	9,602
Recorded investment in loans, Collectively evaluated for impairment	152,593	152,611
Recorded investment in loans Total	161,758	162,213
Consumer [Member]
Reserve for loan losses Individually evaluated for impairment	330	301
Reserve for loan losses Collectively evaluated for impairment	1,709	1,876
Reserve for loan losses Total	2,039	2,177	2,788	2,292
Recorded investment in loans Individually evaluated for impairment	1,938	1,636
Recorded investment in loans, Collectively evaluated for impairment	36,084	37,870
Recorded investment in loans Total	38,022	39,506
Allocated Financing Receivables [Member]
Reserve for loan losses Individually evaluated for impairment	4,429	3,799
Reserve for loan losses Collectively evaluated for impairment	19,461	22,375
Reserve for loan losses Total	23,890	26,174
Recorded investment in loans Individually evaluated for impairment	63,289	68,671
Recorded investment in loans, Collectively evaluated for impairment	813,795	789,493
Recorded investment in loans Total	877,084	858,164
Unallocated [Member]
Reserve for loan losses Total	$ 658	$ 1,087	$ 17	$ 17