REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregated Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Disaggregation of Revenue [Line Items]
Revenues	$ 781.3	$ 677.8	$ 1,513.5	$ 1,334.2
North America
Disaggregation of Revenue [Line Items]
Revenues	744.0	643.8	1,441.4	1,267.1
International
Disaggregation of Revenue [Line Items]
Revenues	37.3	34.0	72.1	67.1
CoStar
Disaggregation of Revenue [Line Items]
Revenues	270.9	253.0	536.0	503.3
CoStar \| North America
Disaggregation of Revenue [Line Items]
Revenues	251.6	237.1	499.2	472.8
CoStar \| International
Disaggregation of Revenue [Line Items]
Revenues	19.3	15.9	36.8	30.5
Information Services
Disaggregation of Revenue [Line Items]
Revenues	39.3	33.4	79.1	66.4
Information Services \| North America
Disaggregation of Revenue [Line Items]
Revenues	35.7	27.9	71.7	55.3
Information Services \| International
Disaggregation of Revenue [Line Items]
Revenues	3.6	5.5	7.4	11.1
Multifamily
Disaggregation of Revenue [Line Items]
Revenues	292.3	264.2	574.8	519.0
Multifamily \| North America
Disaggregation of Revenue [Line Items]
Revenues	292.3	264.2	574.8	519.0
Multifamily \| International
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
LoopNet
Disaggregation of Revenue [Line Items]
Revenues	75.7	69.8	148.5	138.9
LoopNet \| North America
Disaggregation of Revenue [Line Items]
Revenues	72.6	67.2	142.6	133.6
LoopNet \| International
Disaggregation of Revenue [Line Items]
Revenues	3.1	2.6	5.9	5.3
Residential
Disaggregation of Revenue [Line Items]
Revenues	28.4	26.2	55.6	44.8
Residential \| North America
Disaggregation of Revenue [Line Items]
Revenues	17.1	16.2	33.6	24.6
Residential \| International
Disaggregation of Revenue [Line Items]
Revenues	11.3	10.0	22.0	20.2
Other Revenues
Disaggregation of Revenue [Line Items]
Revenues	74.7	31.2	119.5	61.8
Other Revenues \| North America
Disaggregation of Revenue [Line Items]
Revenues	74.7	31.2	119.5	61.8
Other Revenues \| International
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0

COSTAR GROUP, INC. 10-Q