Loans and allowance for credit losses (Details 5) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Allowance for credit losses
Beginning balance	$ 13,544	$ 17,471	$ 14,288	$ 14,227
Charge-offs	(4,260)	(6,948)	(13,511)	(10,274)
Recoveries	181	440	318	620
Net charge-offs	(4,079)	(6,508)	(13,193)	(9,654)
Provision	3,525	5,395	11,895	11,785
Ending balance	12,990	16,358	12,990	16,358
Unallocated Financing Receivables [Member]
Allowance for credit losses
Beginning balance	83	80	22	343
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Net charge-offs	0	0	0	0
Provision	325	(5)	386	(268)
Ending balance	408	75	408	75
Commercial Loan [Member]
Allowance for credit losses
Beginning balance	1,539	2,776	1,498	1,422
Charge-offs	(1,085)	(1,525)	(4,440)	(1,771)
Recoveries	133	380	208	457
Net charge-offs	(952)	(1,145)	(4,232)	(1,314)
Provision	476	1,079	3,797	2,602
Ending balance	1,063	2,710	1,063	2,710
Consumer Loan [Member]
Allowance for credit losses
Beginning balance	581	591	594	637
Charge-offs	(162)	(40)	(177)	(115)
Recoveries	5	21	9	41
Net charge-offs	(157)	(19)	(168)	(74)
Provision	61	33	59	42
Ending balance	485	605	485	605
Residential Real Estate [Member]
Allowance for credit losses
Beginning balance	4,448	5,420	5,014	4,833
Charge-offs	(1,554)	(2,390)	(5,673)	(4,481)
Recoveries	43	34	94	68
Net charge-offs	(1,511)	(2,356)	(5,579)	(4,413)
Provision	1,300	1,120	4,802	3,764
Ending balance	4,237	4,184	4,237	4,184
Commercial Real Estate [Member]
Allowance for credit losses
Beginning balance	3,549	5,280	3,415	3,665
Charge-offs	(745)	(2,265)	(1,435)	(2,493)
Recoveries	0	5	7	5
Net charge-offs	(745)	(2,260)	(1,428)	(2,488)
Provision	1,148	2,231	1,965	4,074
Ending balance	3,952	5,251	3,952	5,251
Construction Loans [Member]
Allowance for credit losses
Beginning balance	3,344	3,324	3,745	3,327
Charge-offs	(714)	(728)	(1,786)	(1,414)
Recoveries	0	0	0	49
Net charge-offs	(714)	(728)	(1,786)	(1,365)
Provision	215	937	886	1,571
Ending balance	$ 2,845	$ 3,533	$ 2,845	$ 3,533