Stock based compensation - Restricted Stock - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Restricted stock disclosures
Stock-based compensation	$ 4,027,000	$ 425,000
Unvested restricted common stock
Shares
Balance, beginning (in shares)	2,578,817
Issued (in shares)		2,897,181
Vested (in shares)	(717,747)	(318,364)
Repurchased (in shares)	(42,809)
Balance, ending (in shares)	1,818,261	2,578,817
Weighted Average Grant Date Fair Value Per Share
Balance, beginning (in dollars per share)	$ 0.77
Issued (in dollars per share)		$ 0.77
Vested (in dolalrs per share)	0.77	0.68
Repurchased (in dollars per share)	0.68
Balance, ending (in dollars per share)	$ 0.76	$ 0.77
Restricted stock disclosures
Unrecognized stock-based compensation expense	$ 32,047,000
Remaining weighted-average remaining vesting period	2 years 4 months 6 days
Employees restricted stock
Restricted stock disclosures
Stock-based compensation	$ 455,000
Non-employee consultants restricted stock
Restricted stock disclosures
Stock-based compensation	$ 2,551,000