Net Loss Per Share Attributable to Compass, Inc. - Schedule of Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Numerator:
Net loss attributable to Compass, Inc.	$ (154.2)	$ (99.8)	$ (443.4)	$ (319.3)
Denominator:
Weighted-average number of shares outstanding used to compute net loss per share attributable to Compass, Inc., basic (in shares)	432,459,739	392,979,596	425,338,530	300,303,624
Weighted-average number of shares outstanding used to compute net loss per share attributable to Compass, Inc., diluted (in shares)	432,459,739	392,979,596	425,338,530	300,303,624
Net loss per share attributable to Compass, Inc., basic (in dollars per share)	$ (0.36)	$ (0.25)	$ (1.04)	$ (1.06)
Net loss per share attributable to Compass, Inc., diluted (in dollars per share)	$ (0.36)	$ (0.25)	$ (1.04)	$ (1.06)