Loans and allowance for credit losses (Details 7) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Allowance for credit losses:
Beginning balance	$ 15,735	$ 13,544	$ 15,991	$ 14,288
Charge-offs	(2,919)	(4,260)	(5,630)	(13,511)
Recoveries	207	181	512	318
Net charge-offs	(2,712)	(4,079)	(5,118)	(13,193)
Provision	2,700	3,525	4,850	11,895
Ending balance	15,723	12,990	15,723	12,990
Unallocated Financing Receivables [Member]
Allowance for credit losses:
Beginning balance	291	83	187	22
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Net charge-offs	0	0	0	0
Provision	(204)	325	(100)	386
Ending balance	87	408	87	408
Commercial Loan [Member]
Allowance for credit losses:
Beginning balance	1,760	1,539	1,682	1,498
Charge-offs	(136)	(1,085)	(223)	(4,440)
Recoveries	50	133	102	208
Net charge-offs	(86)	(952)	(121)	(4,232)
Provision	(458)	476	(345)	3,797
Ending balance	1,216	1,063	1,216	1,063
Consumer Loan [Member]
Allowance for credit losses:
Beginning balance	330	581	407	594
Charge-offs	(13)	(162)	(62)	(177)
Recoveries	7	5	17	9
Net charge-offs	(6)	(157)	(45)	(168)
Provision	(23)	61	(61)	59
Ending balance	301	485	301	485
Residential Real Estate [Member]
Allowance for credit losses:
Beginning balance	4,864	4,448	5,194	5,014
Charge-offs	(1,096)	(1,554)	(1,889)	(5,673)
Recoveries	59	43	298	94
Net charge-offs	(1,037)	(1,511)	(1,591)	(5,579)
Provision	432	1,300	656	4,802
Ending balance	4,259	4,237	4,259	4,237
Commercial Real Estate [Member]
Allowance for credit losses:
Beginning balance	4,374	3,549	4,134	3,415
Charge-offs	(872)	(745)	(1,947)	(1,435)
Recoveries	90	0	93	7
Net charge-offs	(782)	(745)	(1,854)	(1,428)
Provision	1,922	1,148	3,234	1,965
Ending balance	5,514	3,952	5,514	3,952
Construction Loans [Member]
Allowance for credit losses:
Beginning balance	4,116	3,344	4,387	3,745
Charge-offs	(802)	(714)	(1,509)	(1,786)
Recoveries	1	0	2	0
Net charge-offs	(801)	(714)	(1,507)	(1,786)
Provision	1,031	215	1,466	886
Ending balance	$ 4,346	$ 2,845	$ 4,346	$ 2,845