Net Loss Per Share - Anti-dilutive shares excluded from the calculation of diluted net loss per share (Details) - shares shares in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Total shares excluded from net loss per share	18,395	20,805
Options to purchase common stock
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Total shares excluded from net loss per share	9,312	10,740
Common stock warrants
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Total shares excluded from net loss per share	510	1,459
Non-vested restricted stock unit awards
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Total shares excluded from net loss per share	8,573	8,606