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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss used to compute net loss per share, basic									$ (124,827)	$ (128,154)	$ (137,628)
Less: Remeasurement gain on warrant liability											(1,148)
Net loss used to compute net loss per share, diluted									$ (124,827)	$ (128,154)	$ (138,776)
Denominator:
Weighted-average number of shares used in computing net loss per share, basic									69,555	57,848	53,312
Add: incremental shares from assumed exercise of warrants											292
Net loss per share, basic (in dollars per share)	$ (0.46)	$ (0.33)	$ (0.48)	$ (0.54)	$ (0.51)	$ (0.49)	$ (0.62)	$ (0.61)	$ (1.79)	$ (2.22)	$ (2.58)
Net loss per share, diluted (in dollars per share)	$ (0.46)	$ (0.33)	$ (0.48)	$ (0.54)	$ (0.51)	$ (0.49)	$ (0.62)	$ (0.61)	$ (1.79)	$ (2.22)	$ (2.59)