REVENUES - Revenues Disaggregated by Revenue Source and Geographical Segments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Disaggregation of Revenue
Revenues	$ 16,222.1	$ 13,481.9	$ 10,600.2
Variable consideration recognized	30.8
U.S.
Disaggregation of Revenue
Revenues	13,427.1	10,846.3	8,503.4
U.K.
Disaggregation of Revenue
Revenues	2,795.1	2,635.6	2,096.8
Total new and used vehicle sales
Disaggregation of Revenue
Revenues	13,490.4	11,309.3	8,779.8
Total new and used vehicle sales | U.S.
Disaggregation of Revenue
Revenues	11,008.8	8,959.9	6,924.5
Total new and used vehicle sales | U.K.
Disaggregation of Revenue
Revenues	2,481.6	2,349.4	1,855.3
New vehicle retail sales
Disaggregation of Revenue
Revenues	7,452.5	6,504.8	5,428.4
New vehicle retail sales | U.S.
Disaggregation of Revenue
Revenues	6,238.5	5,371.4	4,406.6
New vehicle retail sales | U.K.
Disaggregation of Revenue
Revenues	1,214.0	1,133.3	1,021.8
Used vehicle retail sales
Disaggregation of Revenue
Revenues	5,673.3	4,438.8	3,055.6
Used vehicle retail sales | U.S.
Disaggregation of Revenue
Revenues	4,531.5	3,356.3	2,348.5
Used vehicle retail sales | U.K.
Disaggregation of Revenue
Revenues	1,141.8	1,082.5	707.2
Used vehicle wholesale sales
Disaggregation of Revenue
Revenues	364.6	365.7	295.8
Used vehicle wholesale sales | U.S.
Disaggregation of Revenue
Revenues	238.8	232.2	169.4
Used vehicle wholesale sales | U.K.
Disaggregation of Revenue
Revenues	125.8	133.6	126.4
Parts and service sales
Disaggregation of Revenue
Revenues	2,009.5	1,591.2	1,357.4
Parts and service sales | U.S.
Disaggregation of Revenue
Revenues	1,761.4	1,361.4	1,162.6
Parts and service sales | U.K.
Disaggregation of Revenue
Revenues	248.2	229.8	194.8
Finance, insurance and other, net
Disaggregation of Revenue
Revenues	722.2	581.4	463.0
Variable consideration recognized	30.8	22.3	27.6
Finance, insurance and other, net | U.S.
Disaggregation of Revenue
Revenues	656.9	525.0	416.3
Finance, insurance and other, net | U.K.
Disaggregation of Revenue
Revenues	$ 65.2	$ 56.4	$ 46.6