Revenues - Revenues Disaggregated by Revenue Source and Geographical Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 2,131.2	$ 3,005.7	$ 4,822.0	$ 5,814.1	$ 5,814.1
U.S.
Disaggregation of Revenue [Line Items]
Revenues	1,831.1	2,278.7	3,838.7	4,341.5	4,341.5
U.K.
Disaggregation of Revenue [Line Items]
Revenues	263.5	614.4	854.2	1,256.6	1,256.6
Brazil
Disaggregation of Revenue [Line Items]
Revenues	36.6	112.6	129.1	216.0	216.0
Total new and used vehicle sales
Disaggregation of Revenue [Line Items]
Revenues	1,752.5	2,500.3	3,960.3		4,826.1
Total new and used vehicle sales | U.S.
Disaggregation of Revenue [Line Items]
Revenues	1,487.1	1,858.5	3,092.6		3,527.5
Total new and used vehicle sales | U.K.
Disaggregation of Revenue [Line Items]
Revenues	235.3	543.1	756.2		1,110.5
Total new and used vehicle sales | Brazil
Disaggregation of Revenue [Line Items]
Revenues	30.1	98.6	111.4		188.1
New vehicle retail sales
Disaggregation of Revenue [Line Items]
Revenues	1,062.7	1,565.4	2,404.8	2,979.9	2,979.9
New vehicle retail sales | U.S.
Disaggregation of Revenue [Line Items]
Revenues	915.7	1,188.8	1,904.1		2,220.5
New vehicle retail sales | U.K.
Disaggregation of Revenue [Line Items]
Revenues	127.2	302.2	423.5		620.8
New vehicle retail sales | Brazil
Disaggregation of Revenue [Line Items]
Revenues	19.8	74.4	77.3		138.5
Used vehicle retail sales
Disaggregation of Revenue [Line Items]
Revenues	641.2	838.9	1,420.3	1,658.1	1,658.1
Used vehicle retail sales | U.S.
Disaggregation of Revenue [Line Items]
Revenues	540.9	625.5	1,111.2		1,219.9
Used vehicle retail sales | U.K.
Disaggregation of Revenue [Line Items]
Revenues	92.8	194.0	281.6		397.6
Used vehicle retail sales | Brazil
Disaggregation of Revenue [Line Items]
Revenues	7.5	19.4	27.4		40.7
Used vehicle wholesale sales
Disaggregation of Revenue [Line Items]
Revenues	48.7	96.0	135.2	188.1	188.1
Used vehicle wholesale sales | U.S.
Disaggregation of Revenue [Line Items]
Revenues	30.5	44.3	77.3		87.1
Used vehicle wholesale sales | U.K.
Disaggregation of Revenue [Line Items]
Revenues	15.3	46.9	51.1		92.1
Used vehicle wholesale sales | Brazil
Disaggregation of Revenue [Line Items]
Revenues	2.9	4.8	6.7		8.9
Parts and service sales
Disaggregation of Revenue [Line Items]
Revenues	282.0	378.2	652.6	747.3	747.3
Parts and service sales | U.S.
Disaggregation of Revenue [Line Items]
Revenues	254.2	309.6	558.9		607.2
Parts and service sales | U.K.
Disaggregation of Revenue [Line Items]
Revenues	21.8	56.4	78.3		115.9
Parts and service sales | Brazil
Disaggregation of Revenue [Line Items]
Revenues	5.9	12.2	15.5		24.2
Finance, insurance and other, net
Disaggregation of Revenue [Line Items]
Revenues	96.7	127.3	209.2	240.6	240.6
Variable consideration recognized relating to performance obligations satisfied in previous period	5.4	9.0	9.3	$ 12.2
Finance, insurance and other, net | U.S.
Disaggregation of Revenue [Line Items]
Revenues	89.8	110.5	187.2		206.7
Finance, insurance and other, net | U.K.
Disaggregation of Revenue [Line Items]
Revenues	6.4	15.0	19.7		30.2
Finance, insurance and other, net | Brazil
Disaggregation of Revenue [Line Items]
Revenues	$ 0.6	$ 1.8	$ 2.3		$ 3.8