Stock-Based Compensation - Employee Stock Option and Incentive Plan Activity (Details) - 2008 Plan - Stock options - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Shares
Options Outstanding at beginning of the period (in shares)	10,218,710
Granted (in shares)	2,427,200
Canceled (in shares)	(1,789,698)
Options Outstanding at the end of the period (in shares)	10,856,212	10,218,710
Options Vested and Expected to Vest at the end of the period (in shares)	10,780,023
Options Exercisable at the end of the period (in shares)	6,846,254
Shares Available for Grant (in shares)	8,324,310
Weighted Average Exercise Price Per Share
Options Outstanding at beginning of the period (in dollars per share)	$ 11.14
Granted (in dollars per share)	2.37
Canceled (in dollars per share)	9.76
Options Outstanding at the end of the period (in dollars per share)	9.40	$ 11.14
Options Vested and Expected to Vest at the end of the period (in dollars per share)	9.43
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	6 years 1 month 6 days	6 years 6 months
Options Vested and Expected to Vest at the end of the period	6 years 1 month 6 days
Options Exercisable at the end of the period	4 years 6 months