Net Loss Per Share - Potentially Dilutive Securities (Details) - shares shares in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Potentially dilutive securities
Total potentially dilutive securities excluded from the computation of diluted weighted average shares outstanding (in shares)	65,893	64,516
Options to purchase common stock
Potentially dilutive securities
Total potentially dilutive securities excluded from the computation of diluted weighted average shares outstanding (in shares)	22,056	21,621
Shares subject to repurchase
Potentially dilutive securities
Total potentially dilutive securities excluded from the computation of diluted weighted average shares outstanding (in shares)	130	125
Restricted stock units
Potentially dilutive securities
Total potentially dilutive securities excluded from the computation of diluted weighted average shares outstanding (in shares)	3,207	2,270
Note hedge warrants
Potentially dilutive securities
Total potentially dilutive securities excluded from the computation of diluted weighted average shares outstanding (in shares)	20,250	20,250
Convertible debt securities
Potentially dilutive securities
Total potentially dilutive securities excluded from the computation of diluted weighted average shares outstanding (in shares)	20,250	20,250