REVENUES - Schedule of Revenues Disaggregated by Revenue Source and Geographical Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Disaggregation of Revenue
Total revenues	$ 4,470.5	$ 4,130.0
U.S.
Disaggregation of Revenue
Total revenues	3,645.7	3,342.4
U.K.
Disaggregation of Revenue
Total revenues	824.8	787.7
Total new and used vehicle sales
Disaggregation of Revenue
Total revenues	3,705.4	3,416.6
Total new and used vehicle sales | U.S.
Disaggregation of Revenue
Total revenues	2,979.2	2,721.0
Total new and used vehicle sales | U.K.
Disaggregation of Revenue
Total revenues	726.2	695.6
New vehicle retail sales
Disaggregation of Revenue
Total revenues	2,182.6	1,955.7
New vehicle retail sales | U.S.
Disaggregation of Revenue
Total revenues	1,799.8	1,608.6
New vehicle retail sales | U.K.
Disaggregation of Revenue
Total revenues	382.8	347.1
Used vehicle retail sales
Disaggregation of Revenue
Total revenues	1,416.8	1,348.9
Used vehicle retail sales | U.S.
Disaggregation of Revenue
Total revenues	1,099.5	1,030.1
Used vehicle retail sales | U.K.
Disaggregation of Revenue
Total revenues	317.3	318.8
Used vehicle wholesale sales
Disaggregation of Revenue
Total revenues	106.0	112.0
Used vehicle wholesale sales | U.S.
Disaggregation of Revenue
Total revenues	79.9	82.4
Used vehicle wholesale sales | U.K.
Disaggregation of Revenue
Total revenues	26.2	29.6
Parts and service sales
Disaggregation of Revenue
Total revenues	576.2	548.3
Parts and service sales | U.S.
Disaggregation of Revenue
Total revenues	495.1	473.8
Parts and service sales | U.K.
Disaggregation of Revenue
Total revenues	81.1	74.6
Finance, insurance and other, net
Disaggregation of Revenue
Total revenues	188.9	165.1
Variable consideration recognized relating to performance obligations satisfied in previous period	12.4	4.9
Finance, insurance and other, net | U.S.
Disaggregation of Revenue
Total revenues	171.4	147.6
Finance, insurance and other, net | U.K.
Disaggregation of Revenue
Total revenues	$ 17.5	$ 17.5