Property and equipment Property and equipment - Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 acre	Dec. 31, 2013	Dec. 31, 2012
Property and Equipment [Abstract]
Significant Acquisitions and Disposals, Description	40000
Property, Plant, and Equipment, Additional Disclosures	2.3
Property Subject to or Available for Operating Lease, Net	1.5
Depreciation, Depletion and Amortization	$ 731,000	$ 697,000	$ 684,000