Net Loss Per Share Attributable to Common Stockholders - Calculation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss attributable to common stockholders	$ (128,323)	$ (1,361,371)	$ (62,974)
Denominator:
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	596,264	420,473	127,091
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.22)	$ (3.24)	$ (0.50)
Class A
Numerator:
Net loss attributable to common stockholders	$ (96,499)	$ (459,412)
Denominator:
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	448,392	141,894
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.22)	$ (3.24)
Class B
Numerator:
Net loss attributable to common stockholders	$ (31,824)	$ (901,959)
Denominator:
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	147,872	278,579
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.22)	$ (3.24)