SEGMENT INFORMATION - Reconciliation of Segment Information to Consolidated Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information [Line Items]
Revenues	$ 4,800.9	$ 4,236.7	$ 13,322.5	$ 12,684.1
Cost of sales	(3,998.3)	(3,518.6)	(11,043.8)	(10,485.3)
Depreciation and amortization	21.2	18.9	59.4	55.8
Floor plan interest expense	26.7	22.3	65.6	66.1
INCOME FROM OPERATIONS	(242.6)	(232.7)	(734.3)	(596.0)
Asset impairments	(11.7)	0.0	(26.0)	(135.4)
Other interest expense, net	(51.3)	(45.7)	(135.0)	(134.9)
Gain on divestitures, net	35.7	5.0	45.8	8.6
Income before income taxes	200.2	169.7	579.5	403.6
New vehicle
Segment Reporting Information [Line Items]
Revenues	2,528.9	2,163.5	6,970.9	6,392.6
Cost of sales	(2,367.9)	(2,013.1)	(6,506.8)	(5,924.4)
Used vehicle
Segment Reporting Information [Line Items]
Revenues	1,412.2	1,294.7	3,933.8	3,959.6
Cost of sales	(1,347.0)	(1,235.3)	(3,735.0)	(3,767.3)
Parts and service
Segment Reporting Information [Line Items]
Revenues	659.4	593.1	1,848.5	1,764.3
Cost of sales	(270.2)	(256.0)	(761.9)	(753.2)
Finance and insurance, net
Segment Reporting Information [Line Items]
Revenues	200.3	185.4	569.3	567.5
Cost of sales	(13.2)	(14.2)	(40.1)	(40.5)
Dealerships
Segment Reporting Information [Line Items]
Revenues	4,719.8	4,159.7	13,077.8	12,457.6
TCA
Segment Reporting Information [Line Items]
Revenues	81.1	77.0	244.7	226.5
Operating Segments
Segment Reporting Information [Line Items]
Revenues	4,867.2	4,294.3	13,525.4	12,851.1
INCOME FROM OPERATIONS	(233.5)	(210.1)	(713.6)	(661.5)
Operating Segments \| Dealerships
Segment Reporting Information [Line Items]
Revenues	4,786.2	4,217.3	13,280.7	12,624.6
Personnel costs	339.0	307.7	966.8	939.2
Rent and related expenses	39.5	37.2	89.3	103.1
Advertising	18.7	16.0	51.0	47.8
Other selling, general and administrative expense	133.1	108.4	362.1	332.1
Other segment items	0.0	0.0	0.0	0.0
Depreciation and amortization	21.2	18.8	59.3	55.5
Floor plan interest expense	26.7	22.3	65.6	66.1
INCOME FROM OPERATIONS	(213.9)	(192.4)	(654.4)	(606.2)
Operating Segments \| Dealerships \| New vehicle
Segment Reporting Information [Line Items]
Cost of sales	(2,367.9)	(2,013.1)	(6,506.8)	(5,924.4)
Operating Segments \| Dealerships \| Used vehicle
Segment Reporting Information [Line Items]
Cost of sales	(1,347.0)	(1,235.3)	(3,735.0)	(3,767.3)
Operating Segments \| Dealerships \| Parts and service
Segment Reporting Information [Line Items]
Cost of sales	(279.2)	(266.2)	(790.4)	(782.9)
Operating Segments \| Dealerships \| Finance and insurance, net
Segment Reporting Information [Line Items]
Cost of sales	0.0	0.0	0.0	0.0
Operating Segments \| TCA
Segment Reporting Information [Line Items]
Revenues	81.1	77.0	244.7	226.5
Personnel costs	0.0	0.0	0.0	0.0
Rent and related expenses	0.0	0.0	0.0	0.0
Advertising	0.0	0.0	0.0	0.0
Other selling, general and administrative expense	0.0	0.0	0.0	0.0
Other segment items	1.9	1.6	5.5	5.1
Depreciation and amortization	0.0	0.1	0.2	0.3
Floor plan interest expense	0.0	0.0	0.0	0.0
INCOME FROM OPERATIONS	(19.6)	(17.8)	(59.2)	(55.3)
Operating Segments \| TCA \| New vehicle
Segment Reporting Information [Line Items]
Cost of sales	0.0	0.0	0.0	0.0
Operating Segments \| TCA \| Used vehicle
Segment Reporting Information [Line Items]
Cost of sales	0.0	0.0	0.0	0.0
Operating Segments \| TCA \| Parts and service
Segment Reporting Information [Line Items]
Cost of sales	0.0	0.0	0.0	0.0
Operating Segments \| TCA \| Finance and insurance, net
Segment Reporting Information [Line Items]
Cost of sales	(59.6)	(57.5)	(179.8)	(165.8)
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	(66.4)	(57.6)	(202.9)	(167.0)
Cost of sales	55.3	53.4	168.2	154.9
INCOME FROM OPERATIONS	(5.9)	0.2	(18.9)	3.7
Deferral of SG&A expense (related to capitalized contract costs offset by amortization)	5.1	4.3	15.9	15.7
Intersegment Eliminations \| Dealerships
Segment Reporting Information [Line Items]
Revenues	(66.4)	(57.6)	(202.9)	(167.0)
Intersegment Eliminations \| TCA
Segment Reporting Information [Line Items]
Revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0

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