Commitments and contingencies (Details) (USD $)	1 Months Ended	12 Months Ended			1 Months Ended
	Feb. 28, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	May 31, 2014 item sqft
Commitments and contingencies
Term period of lease	63 months
Operating lease period for monthly rent increasing in fixed monthly rent	12 months
Operating lease period after which monthly rent increase	3 months
Operating leases
Total		$ 552,000
Less than 1 Year		133,000
1-3 Years		251,000
3-5 Years		168,000
Total
Total		552,000
Less than 1 Year		133,000
1-3 Years		251,000
3-5 Years		168,000
Rent expense		300,000	300,000	300,000
Third amendment to the operating lease agreement for laboratory facility in Plymouth, Michigan
Commitments and contingencies
Adjusted rentable laboratory space (in square feet)					3,045
Adjusted proportionate share of landlord's expenses and taxes (as a percent)					7.40%
Number of terms which Company may exercise to renew lease					1
Lease renewal term					3 years
Annual base rent under new lease agreement					$ 37,000