SEGMENT INFORMATION - Reportable Segment Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Revenue	$ 14,802.7	$ 15,433.8	$ 9,837.7
Gross profit	2,755.8	3,100.6	1,902.2
Depreciation and amortization	67.7	69.0	41.9
Selling, general, and administrative	1,617.4	1,763.4	1,073.9
Floor plan interest expense	9.6	8.4	8.2
Other interest expense, net	156.1	152.2	$ 93.9
Total interest expense	165.7	160.6
Capital expenditures	142.3	94.6
Total assets	10,159.4	8,021.4
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue	(181.5)	(153.0)
Gross profit	7.6	10.8
Depreciation and amortization	0.0	0.0
Selling, general, and administrative	(28.5)	(30.0)
Floor plan interest expense	0.0	0.0
Other interest expense, net	0.0	0.0
Total interest expense	0.0	0.0
Capital expenditures	0.0	0.0
Total assets	46.1	(18.6)
Dealerships | Operating Segments
Segment Reporting Information [Line Items]
Revenue	14,699.0	15,341.1
Gross profit	2,671.1	3,036.0
Depreciation and amortization	67.1	68.2
Selling, general, and administrative	1,638.5	1,786.3
Floor plan interest expense	9.6	8.4
Other interest expense, net	156.1	152.2
Total interest expense	165.7	160.6
Capital expenditures	142.3	94.6
Total assets	9,199.4	7,170.8
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Revenue	285.2	245.8
Gross profit	77.1	53.8
Depreciation and amortization	0.7	0.8
Selling, general, and administrative	7.4	7.0
Floor plan interest expense	0.0	0.0
Other interest expense, net	0.0	0.0
Total interest expense	0.0	0.0
Capital expenditures	0.0	0.0
Total assets	$ 913.9	$ 869.2