Stockholders' Equity - Options Outstanding (Details) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Number of Shares
Balance at the beginning of the period (in shares)	8,624,223
Granted (in shares)	2,600,625
Exercised (in shares)	(88,048)
Forfeited/Cancelled (in shares)	(739,557)
Balance at the end of the period (in shares)	10,397,243	8,624,223
Vested or expected to vest at the end of the period (in shares)	10,397,243
Exercisable at the end of the period (in shares)	4,186,575
Weighted-Average Exercise Price
Balance at the beginning of the period (in dollars per share)	$ 5.22
Granted (in dollars per share)	1.79
Exercised (in dollars per share)	0.65
Forfeitures/Expirations (in dollars per share)	5.14
Balance at the end of the period (in dollars per share)	4.40	$ 5.22
Vested or expected to vest at the end of the period (in dollars per share)	4.40
Weighted average exercise price (in dollars per share)	$ 5.49
Weighted Average Remaining Contractual Term
Options Outstanding at the end of the period	8 years 4 days	7 years 2 months 12 days
Vested or expected to vest at the end of the period	8 years 4 days
Exercisable at the end of the period	6 years 2 months 23 days