Condensed Consolidated Statements of Stockholders’ Equity - USD ($) $ in Millions	Total	2021 Agent Equity Program	2022 Agent Equity Program	Common Stock	Common Stock 2021 Agent Equity Program	Common Stock 2022 Agent Equity Program	Additional Paid-in Capital	Additional Paid-in Capital 2021 Agent Equity Program	Additional Paid-in Capital 2022 Agent Equity Program	Accumulated Deficit	Total Compass, Inc. Stockholders’ Equity	Total Compass, Inc. Stockholders’ Equity 2021 Agent Equity Program	Total Compass, Inc. Stockholders’ Equity 2022 Agent Equity Program	Non-controlling Interest
Beginning balance (in shares) at Dec. 31, 2021				409,267,751
Beginning balance at Dec. 31, 2021	$ 847.6						$ 2,438.8			$ (1,595.0)	$ 843.8			$ 3.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(443.5)									(443.4)	(443.4)			(0.1)
Issuance of common stock in connection with acquisitions (in shares)				335,252
Issuance of common stock in connection with acquisitions	2.0						2.0				2.0
Issuance of common stock upon exercise of stock options (in shares)				3,949,244
Issuance of common stock upon exercise of stock options	8.6						8.6				8.6
Issuance of common stock upon settlement of RSUs, net of taxes withheld (in shares)				6,328,555
Issuance of common stock upon settlement of RSUs, net of taxes withheld	(19.5)						(19.5)				(19.5)
Vesting of early exercised stock options	5.3						5.3				5.3
Issuance of common stock in connection with the Agent Equity Program (in shares)					13,608,896
Issuance of common stock in connection with the Agent Equity Program		$ 100.0						$ 100.0				$ 100.0
Issuance of common stock under the Employee Stock Purchase Plan (in shares)				578,921
Issuance of common stock under the Employee Stock Purchase Plan	2.3						2.3				2.3
Stock-based compensation	138.8						138.8				138.8
Ending balance (in shares) at Sep. 30, 2022				434,068,619
Ending balance at Sep. 30, 2022	641.6						2,676.3			(2,038.4)	637.9			3.7
Beginning balance (in shares) at Jun. 30, 2022				429,957,017
Beginning balance at Jun. 30, 2022	755.8						2,636.4			(1,884.2)	752.2			3.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(154.1)									(154.2)	(154.2)			0.1
Issuance of common stock in connection with acquisitions (in shares)				211,400
Issuance of common stock in connection with acquisitions	1.2						1.2				1.2
Issuance of common stock upon exercise of stock options (in shares)				417,056
Issuance of common stock upon exercise of stock options	0.9						0.9				0.9
Issuance of common stock upon settlement of RSUs, net of taxes withheld (in shares)				2,904,225
Issuance of common stock upon settlement of RSUs, net of taxes withheld	(5.7)						(5.7)				(5.7)
Vesting of early exercised stock options	3.1						3.1				3.1
Issuance of common stock under the Employee Stock Purchase Plan (in shares)				578,921
Issuance of common stock under the Employee Stock Purchase Plan	2.3						2.3				2.3
Stock-based compensation	38.1						38.1				38.1
Ending balance (in shares) at Sep. 30, 2022				434,068,619
Ending balance at Sep. 30, 2022	$ 641.6						2,676.3			(2,038.4)	637.9			3.7
Beginning balance (in shares) at Dec. 31, 2022	438,098,194			438,098,194
Beginning balance at Dec. 31, 2022	$ 520.7						2,713.6			(2,196.5)	517.1			3.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(236.3)									(237.6)	(237.6)			1.3
Issuance of common stock in connection with acquisitions (in shares)				3,215,610
Issuance of common stock in connection with acquisitions	$ 10.6						10.6				10.6
Issuance of common stock upon exercise of stock options (in shares)	2,745,170			2,745,170
Issuance of common stock upon exercise of stock options	$ 4.2						4.2				4.2
Issuance of common stock upon settlement of RSUs, net of taxes withheld (in shares)				9,919,619
Issuance of common stock upon settlement of RSUs, net of taxes withheld	(17.9)						(17.9)				(17.9)
Vesting of early exercised stock options	0.4						0.4				0.4
Issuance of common stock in connection with the Agent Equity Program (in shares)						14,147,480
Issuance of common stock in connection with the Agent Equity Program			$ 53.3						$ 53.3				$ 53.3
Issuance of common stock under the Employee Stock Purchase Plan (in shares)				759,835
Issuance of common stock under the Employee Stock Purchase Plan	2.5						2.5				2.5
Issuance of common stock in connection with the Strategic Transaction (in shares)				8,957,910
Issuance of common stock in connection with the Strategic Transaction	30.0						30.0				30.0
Stock-based compensation	111.4						111.4				111.4
Other activity related to non-controlling interests	$ (1.5)													(1.5)
Ending balance (in shares) at Sep. 30, 2023	477,843,818			477,843,818
Ending balance at Sep. 30, 2023	$ 477.4						2,908.1			(2,434.1)	474.0			3.4
Beginning balance (in shares) at Jun. 30, 2023				462,987,617
Beginning balance at Jun. 30, 2023	451.7						2,842.3			(2,394.7)	447.6			4.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(39.2)									(39.4)	(39.4)			0.2
Issuance of common stock in connection with acquisitions (in shares)				637,406
Issuance of common stock in connection with acquisitions	2.4						2.4				2.4
Issuance of common stock upon exercise of stock options (in shares)				648,062
Issuance of common stock upon exercise of stock options	1.3						1.3				1.3
Issuance of common stock upon settlement of RSUs, net of taxes withheld (in shares)				4,221,908
Issuance of common stock upon settlement of RSUs, net of taxes withheld	(7.6)						(7.6)				(7.6)
Issuance of common stock under the Employee Stock Purchase Plan (in shares)				390,915
Issuance of common stock under the Employee Stock Purchase Plan	1.1						1.1				1.1
Issuance of common stock in connection with the Strategic Transaction (in shares)				8,957,910
Issuance of common stock in connection with the Strategic Transaction	30.0						30.0				30.0
Stock-based compensation	38.6						38.6				38.6
Other activity related to non-controlling interests	$ (0.9)													(0.9)
Ending balance (in shares) at Sep. 30, 2023	477,843,818			477,843,818
Ending balance at Sep. 30, 2023	$ 477.4						$ 2,908.1			$ (2,434.1)	$ 474.0			$ 3.4