Net Loss per Share - Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss used to compute net loss per share, basic									$ (128,154)	$ (137,628)	$ (100,327)
Less: Remeasurement gain on warrant liability										(1,148)
Net loss used to compute net loss per share, diluted									$ (128,154)	$ (138,776)	$ (100,327)
Denominator:
Weighted-average number of shares used in computing net loss per share, basic									57,848	53,312	51,667
Less: weighted-average unvested shares subject to repurchase											(91)
Add: incremental shares from assumed exercise of warrants										292
Net loss per share, basic (in dollars per share)	$ (0.51)	$ (0.49)	$ (0.62)	$ (0.61)	$ (0.88)	$ (0.52)	$ (0.55)	$ (0.63)	$ (2.22)	$ (2.58)	$ (1.95)
Net loss per share, diluted (in dollars per share)	$ (0.51)	$ (0.49)	$ (0.62)	$ (0.61)	$ (0.90)	$ (0.52)	$ (0.55)	$ (0.65)	$ (2.22)	$ (2.59)	$ (1.95)