Loans and allowance for credit losses (Details 5) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Allowance for credit losses:
Beginning balance	$ 12,990	$ 16,358	$ 14,288	$ 14,227
Charge-offs	(6,391)	(6,801)	(19,902)	(17,075)
Recoveries	156	333	474	953
Net charge-offs	(6,235)	(6,468)	(19,428)	(16,122)
Provision	6,200	3,650	18,095	15,435
Ending balance	12,955	13,540	12,955	13,540
Unallocated Financing Receivables [Member]
Allowance for credit losses:
Beginning balance	408	75	22	343
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Net charge-offs	0	0	0	0
Provision	(408)	171	(22)	(97)
Ending balance	0	246	0	246
Commercial Loan [Member]
Allowance for credit losses:
Beginning balance	1,063	2,710	1,498	1,422
Charge-offs	(73)	(1,519)	(4,513)	(3,290)
Recoveries	22	47	230	504
Net charge-offs	(51)	(1,472)	(4,283)	(2,786)
Provision	524	315	4,321	2,917
Ending balance	1,536	1,553	1,536	1,553
Consumer Loan [Member]
Allowance for credit losses:
Beginning balance	485	605	594	637
Charge-offs	(31)	(33)	(208)	(148)
Recoveries	8	19	17	60
Net charge-offs	(23)	(14)	(191)	(88)
Provision	(158)	19	(99)	61
Ending balance	304	610	304	610
Residential Real Estate [Member]
Allowance for credit losses:
Beginning balance	4,237	4,184	5,014	4,833
Charge-offs	(2,323)	(2,859)	(7,996)	(7,340)
Recoveries	5	38	99	106
Net charge-offs	(2,318)	(2,821)	(7,897)	(7,234)
Provision	2,268	2,679	7,070	6,443
Ending balance	4,187	4,042	4,187	4,042
Commercial Real Estate [Member]
Allowance for credit losses:
Beginning balance	3,952	5,251	3,415	3,665
Charge-offs	(742)	(1,385)	(2,177)	(3,878)
Recoveries	116	229	123	234
Net charge-offs	(626)	(1,156)	(2,054)	(3,644)
Provision	275	(404)	2,240	3,670
Ending balance	3,601	3,691	3,601	3,691
Construction Loans [Member]
Allowance for credit losses:
Beginning balance	2,845	3,533	3,745	3,327
Charge-offs	(3,222)	(1,005)	(5,008)	(2,419)
Recoveries	5	0	5	49
Net charge-offs	(3,217)	(1,005)	(5,003)	(2,370)
Provision	3,699	870	4,585	2,441
Ending balance	$ 3,327	$ 3,398	$ 3,327	$ 3,398