Commitments and contingencies - Operating Leases - (Details)	1 Months Ended		12 Months Ended
	Jan. 31, 2016 USD ($) ft²	Mar. 31, 2014 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Commitments and Contingencies
Number of renewal options | item		2
Term of each renewal option		5 years
Leasehold incentive received from landlord		$ 1,250,000
Rent expense			$ 686,000	$ 910,000	$ 48,000
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2016			1,170,000
2017			1,192,000
2018			1,214,000
2019			1,184,000
Total minimum lease payments			$ 4,760,000
Area of additional facility leased | ft²	26,000
Estimated lease amendment related payment	$ 25,000,000