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DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Millions			12 Months Ended
	Dec. 11, 2023 USD ($) collisionCenter	Dec. 17, 2021 USD ($)	Dec. 31, 2024 USD ($) reportingUnit reportableSegment shares	Dec. 31, 2023 USD ($) shares	Dec. 31, 2022 USD ($) shares	May 15, 2024 USD ($)	May 14, 2024 USD ($)	Sep. 30, 2023	Nov. 19, 2021	Feb. 19, 2020
Accounting Policies [Line Items]
Restricted investments, fair value			$ 3.6
Repurchase or retirement of previously repurchased common stock			0.0	$ 0.0	$ 0.0
Advertising			61.8	47.5	50.1
Advertising credits and volume discounts			40.7	36.5	35.5
Investments			348.6
Total debt outstanding			$ 3,158.5	3,230.1
Revenues by major brand, disclosure percentage threshold			5.00%
Number of reportable segments \| reportableSegment			2
Goodwill			$ 2,044.7	$ 2,009.0	$ 1,783.4
Number of Reporting Units \| reportingUnit			3
Treasury stock, shares, retired (in shares) \| shares			830,297	1,370,371	1,580,826
Stock repurchase program, increase (decrease) in authorized amount							$ 256.2
Stock repurchase program, authorized amount						$ 400.0
Remaining authorized repurchase amount			$ 275.9
Treasury Stock
Accounting Policies [Line Items]
Repurchase or retirement of previously repurchased common stock			$ (183.0)	$ (267.7)	$ (287.4)
Purchase of treasury shares (in shares) \| shares			830,297	1,316,167	1,635,030
LHM Acquisition
Accounting Policies [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets		$ 5.6
Acquired intangible asset, useful life		5 years
Jim Koons Dealerships
Accounting Policies [Line Items]
Number of new vehicle dealerships acquired (in new vehicle dealerships) \| collisionCenter	20
Number of collision centers acquired (in collision centers) \| collisionCenter	6
Goodwill	$ 272.4
Aggregate purchase price	1,504.5
Assets held for sale	100.9
New Floor Plan Facility \| Jim Koons Dealerships
Accounting Policies [Line Items]
Aggregate purchase price	$ 256.1
Senior Notes
Accounting Policies [Line Items]
Carrying Value of Collateralized Related Real Estate			$ 2,260.6	$ 2,259.7
Senior Notes \| 4.50% Senior Notes due 2028
Accounting Policies [Line Items]
Stated interest rate of debt instrument			4.50%					4.50%		4.50%
Carrying Value of Collateralized Related Real Estate			$ 403.4	402.8
Senior Notes \| 4.75% Senior Notes due 2030
Accounting Policies [Line Items]
Stated interest rate of debt instrument			4.75%					4.75%		4.75%
Carrying Value of Collateralized Related Real Estate			$ 442.8	442.2
Senior Notes \| 5.00% Senior Notes due 2032
Accounting Policies [Line Items]
Stated interest rate of debt instrument			5.00%						5.00%
Carrying Value of Collateralized Related Real Estate			$ 593.0	592.3
Senior Notes \| 4.625% Senior Notes due 2029
Accounting Policies [Line Items]
Stated interest rate of debt instrument			4.625%						4.625%
Carrying Value of Collateralized Related Real Estate			$ 791.9	$ 790.4
Minimum
Accounting Policies [Line Items]
Loaner vehicle period of use before sale (in months)			6 months
Maximum
Accounting Policies [Line Items]
Loaner vehicle period of use before sale (in months)			12 months