Stock Based Compensation - Assumptions (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Weighted average assumptions for estimated fair value of employee stock options
Total unrecognized compensation expense, net of estimated forfeitures	$ 16.8		$ 16.8
Period to recognize total unrecognized compensation expense			3 years
Employee stock options
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	5 years 10 months 24 days	5 years 10 months 24 days	6 years 1 month 6 days	6 years
Expected volatility (as a percent)	76.10%	75.60%	75.80%	76.40%
Risk-free interest rate (as a percent)	1.36%	1.60%	1.53%	1.61%
Employee Stock Purchase Plan | Minimum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	6 months	6 months	6 months	6 months
Expected volatility (as a percent)	67.80%	70.60%	67.80%	70.60%
Risk-free interest rate (as a percent)	0.47%	0.06%	0.47%	0.06%
Employee Stock Purchase Plan | Maximum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	2 years	2 years	2 years	2 years
Expected volatility (as a percent)	79.60%	82.90%	79.60%	82.90%
Risk-free interest rate (as a percent)	0.93%	0.46%	0.93%	0.46%