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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss used to compute net loss per share, basic									$ (136,314)	$ (95,765)	$ (70,275)
Less: Remeasurement gain on warrant liability									(1,148)
Net loss attributable to common shares, diluted									$ (137,462)	$ (95,765)	$ (70,275)
Denominator:
Weighted-average shares outstanding									53,312	51,667	27,687
Less: weighted-average unvested shares subject to repurchase										(91)	(1,483)
Weighted-average number of shares used in computing net loss per share, basic									53,312	51,576	26,204
Add: incremental shares from assumed exercise of warrants									292
Weighted-average number of shares used in computing net loss per share, diluted									53,604	51,576	26,204
Net loss per share, basic (in dollars per share)	$ (0.84)	$ (0.51)	$ (0.52)	$ (0.68)	$ (0.72)	$ (0.50)	$ (0.46)	$ (0.17)	$ (2.56)	$ (1.86)	$ (2.68)
Net loss per share, diluted (in dollars per share)	$ (0.87)	$ (0.51)	$ (0.52)	$ (0.70)	$ (0.72)	$ (0.50)	$ (0.46)	$ (0.17)	$ (2.56)	$ (1.86)	$ (2.68)