REVENUES - Schedule of Revenues Disaggregated by Revenue Source and Geographical Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Disaggregation of Revenue
Total revenues	$ 5,407.1	$ 5,505.3
U.S.
Disaggregation of Revenue
Total revenues	3,763.8	3,921.8
U.K.
Disaggregation of Revenue
Total revenues	1,643.3	1,583.5
Total new and used vehicle sales
Disaggregation of Revenue
Total revenues	4,486.8	4,587.0
Total new and used vehicle sales \| U.S.
Disaggregation of Revenue
Total revenues	3,064.1	3,205.0
Total new and used vehicle sales \| U.K.
Disaggregation of Revenue
Total revenues	1,422.7	1,381.9
New vehicle retail sales
Disaggregation of Revenue
Total revenues	2,562.4	2,680.0
New vehicle retail sales \| U.S.
Disaggregation of Revenue
Total revenues	1,852.0	1,968.7
New vehicle retail sales \| U.K.
Disaggregation of Revenue
Total revenues	710.4	711.2
Used vehicle retail sales
Disaggregation of Revenue
Total revenues	1,774.9	1,755.4
Used vehicle retail sales \| U.S.
Disaggregation of Revenue
Total revenues	1,117.5	1,144.3
Used vehicle retail sales \| U.K.
Disaggregation of Revenue
Total revenues	657.4	611.1
Used vehicle wholesale sales
Disaggregation of Revenue
Total revenues	149.5	151.6
Used vehicle wholesale sales \| U.S.
Disaggregation of Revenue
Total revenues	94.6	92.0
Used vehicle wholesale sales \| U.K.
Disaggregation of Revenue
Total revenues	54.9	59.6
Parts and service sales
Disaggregation of Revenue
Total revenues	704.4	692.1
Parts and service sales \| U.S.
Disaggregation of Revenue
Total revenues	527.2	531.3
Parts and service sales \| U.K.
Disaggregation of Revenue
Total revenues	177.3	160.8
Finance, insurance and other, net
Disaggregation of Revenue
Total revenues	215.9	226.2
Variable consideration recognized	(0.3)	10.4
Finance, insurance and other, net \| U.S.
Disaggregation of Revenue
Total revenues	172.6	185.5
Finance, insurance and other, net \| U.K.
Disaggregation of Revenue
Total revenues	$ 43.3	$ 40.8