ACQUISITIONS AND DIVESTITURES - Narrative (Details) $ in Millions					1 Months Ended	12 Months Ended
	Dec. 11, 2023 USD ($) collisionCenter	Dec. 31, 2021 USD ($)	Dec. 17, 2021 USD ($) collisionCenter usedVehicleStore	May 20, 2021 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) dealership_location franchise collisionCenter	Dec. 31, 2022 USD ($) franchise dealership_location center	Dec. 31, 2021 USD ($) franchise dealership_location
Business Acquisition [Line Items]
Goodwill		$ 2,271.7			$ 2,009.0	$ 2,009.0	$ 1,783.4	$ 2,271.7
Acquisition related costs						4.1
Revenue						14,802.7	15,433.8	9,837.7
Net income						602.5	997.3	532.4
OPERATING LEASE RIGHT-OF-USE ASSETS					241.8	241.8	$ 235.4
Operating lease, liability					250.5	$ 250.5
Number of dealership locations, sold (in dealership locations) | dealership_location						1	16
Number of collision centers, sold (in collision centers) | collisionCenter						3
Gain on divestitures						$ 13.5	$ 207.1	8.0
Asset Purchase Agreement
Business Acquisition [Line Items]
OPERATING LEASE RIGHT-OF-USE ASSETS				$ 99.5
Operating lease, liability				99.5
Operating lease				$ 217.1
Colorado | Disposed of by Sale
Business Acquisition [Line Items]
Number of franchises, sold (in franchises) | franchise							3
Number of dealership locations, sold (in dealership locations) | dealership_location							3
Number of collision centers, sold (in collision centers) | center							1
MISSOURI | Disposed of by Sale
Business Acquisition [Line Items]
Number of franchises, sold (in franchises) | franchise							1
Number of dealership locations, sold (in dealership locations) | dealership_location							1
WASHINGTON | Disposed of by Sale
Business Acquisition [Line Items]
Number of franchises, sold (in franchises) | franchise							2
Number of dealership locations, sold (in dealership locations) | dealership_location							2
NEW MEXICO | Disposed of by Sale
Business Acquisition [Line Items]
Number of franchises, sold (in franchises) | franchise							1
Number of dealership locations, sold (in dealership locations) | dealership_location							1
NORTH CAROLINA | Disposed of by Sale
Business Acquisition [Line Items]
Number of franchises, sold (in franchises) | franchise							11
Number of dealership locations, sold (in dealership locations) | dealership_location							9
Number of collision centers, sold (in collision centers) | center							2
Virginia
Business Acquisition [Line Items]
Gain on divestitures								$ 8.0
Virginia | Disposed of by Sale
Business Acquisition [Line Items]
Number of franchises, sold (in franchises) | franchise								1
Number of dealership locations, sold (in dealership locations) | dealership_location								1
Texas | Disposed of by Sale
Business Acquisition [Line Items]
Number of franchises, sold (in franchises) | franchise						1
Number of dealership locations, sold (in dealership locations) | dealership_location						1
Dealerships
Business Acquisition [Line Items]
Goodwill		1,561.4			1,472.4	$ 1,472.4	$ 1,246.8	$ 1,561.4
Jim Koons Dealerships
Business Acquisition [Line Items]
Number of new vehicle dealerships acquired (in new vehicle dealerships) | collisionCenter	20
Number of collision centers acquired (in collision centers) | collisionCenter	6
Aggregate purchase price	$ 1,500.0
Assets held for sale	103.8
Intangible franchise rights	429.0
Goodwill	231.7
Revenue					168.2
Net income					7.0
Payments to acquire businesses, gross	936.8
Jim Koons Dealerships | New Floor Plan Facility
Business Acquisition [Line Items]
Aggregate purchase price	$ 256.1
LHM Acquisition
Business Acquisition [Line Items]
Number of new vehicle dealerships acquired (in new vehicle dealerships) | collisionCenter			54
Number of used cars stores acquired (in used cars stores) | usedVehicleStore			7
Number of collision centers acquired (in collision centers) | collisionCenter			11
Aggregate purchase price			$ 3,193.9
Intangible franchise rights			1,309.7
Goodwill			1,205.3
Transaction costs					4.9	4.9
Revenue		256.4
Net income		$ 15.7
Payments to acquire businesses, gross			3,480.0
Payable to sellers			6.0
LHM Acquisition | Insurance
Business Acquisition [Line Items]
Goodwill			536.6
LHM Acquisition | Dealerships
Business Acquisition [Line Items]
Goodwill			$ 668.7
Series of Individually Immaterial Business Acquisitions
Business Acquisition [Line Items]
Aggregate purchase price						485.7
Intangible franchise rights					228.2	228.2
Goodwill					$ 110.5	110.5
Payments to acquire businesses, gross						455.1
Payable to sellers						9.6
Purchase price holdbacks						$ 21.0
Series of Individually Immaterial Business Acquisitions | Colorado
Business Acquisition [Line Items]
Number of franchises acquired (in franchises) | franchise						11
Number of dealership locations acquired (in dealership locations) | dealership_location						10
Goodwill and manufacturer franchise rights, period for federal and state tax deductions							15 years
Series of Individually Immaterial Business Acquisitions | Indiana
Business Acquisition [Line Items]
Number of franchises acquired (in franchises) | franchise						3
Number of dealership locations acquired (in dealership locations) | dealership_location						1