Stock-Based Compensation - Summary of Plans (Details) - Stock options - $ / shares	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Shares
Options Outstanding at beginning of the period (in shares)	10,856,212
Granted (in shares)	5,046,867
Exercised (in shares)	(104,500)
Canceled (in shares)	(1,832,387)
Options Outstanding at the end of the period (in shares)	13,966,192	10,856,212
Options Vested and Expected to Vest at the end of the period (in shares)	13,544,093
Options Exercisable at the end of the period (in shares)	6,482,436
Shares Available for Grant Under the 2008 Plan	4,370,007
Weighted Average Exercise Price Per Share
Options Outstanding at beginning of the period (in dollars per share)	$ 9.40
Granted (in dollars per share)	0.61
Exercised (in dollars per share)	2.80
Canceled (in dollars per share)	8.19
Options Outstanding at the end of the period (in dollars per share)	6.43	$ 9.40
Options Vested and Expected to Vest at the end of the period (in dollars per share)	6.59
Options Exercisable at the end of the period (in dollars per share)	$ 11.18
Weighted Average Remaining Contractual Term (In Years)
Options Outstanding at the end of the period	7 years 4 months 24 days	6 years 1 month 6 days
Options Vested and Expected to Vest at the end of the period	7 years 3 months 18 days
Options Exercisable at the end of the period	5 years
Weighted average grant-date fair value (in dollars per share)	$ 0.44