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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 |  |  |  |  |  |  |

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