Stock-Based Compensation - Summary of Plans (Details) - Stock options - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Shares
Options Outstanding at beginning of the period (in shares)	10,218,710
Granted (in shares)	2,309,900
Canceled (in shares)	(945,792)
Options Outstanding at the end of the period (in shares)	11,582,818	10,218,710
Options Vested and Expected to Vest at the end of the period (in shares)	11,460,583
Options Exercisable at the end of the period (in shares)	7,360,799
Shares Available for Grant Under the 2008 Plan	7,780,663
Weighted Average Exercise Price Per Share
Options Outstanding at beginning of the period (in dollars per share)	$ 11.14
Granted (in dollars per share)	2.34
Canceled (in dollars per share)	9.21
Options Outstanding at the end of the period (in dollars per share)	9.54	$ 11.14
Options Vested and Expected to Vest at the end of the period (in dollars per share)	9.59
Options Exercisable at the end of the period (in dollars per share)	$ 11.05
Weighted Average Remaining Contractual Term (In Years)
Options Outstanding at the end of the period	5 years 10 months 24 days	6 years 6 months
Options Vested and Expected to Vest at the end of the period	5 years 9 months 18 days
Options Exercisable at the end of the period	4 years 1 month 6 days
Weighted average grant-date fair value (in dollars per share)	$ 1.57