Basic and Diluted Net Loss per Common Share (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net loss attributable to common stockholders	$ (65,664)	$ (88,948)
Denominator:
Weighted-average common shares outstanding - basic and diluted (in shares)	264,369,317	256,968,248
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	23,008,000	29,664,000
Options to purchase common stock
Denominator:
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	15,084,000	19,086,000
Convertible notes
Denominator:
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	458,000	462,000
Outstanding warrants, convertible to common stock
Denominator:
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	0	2,554,000
Unvested restricted stock units
Denominator:
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	7,466,000	7,562,000