Revenues - Revenues Disaggregated by Revenue Source and Geographical Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 3,039.6	$ 3,118.3	$ 7,861.7	$ 8,932.4	$ 8,932.4
U.S.
Disaggregation of Revenue [Line Items]
Revenues	2,244.6	2,422.8	6,083.3	6,764.3	6,764.3
U.K.
Disaggregation of Revenue [Line Items]
Revenues	740.8	584.6	1,595.0	1,841.2	1,841.2
Brazil
Disaggregation of Revenue [Line Items]
Revenues	54.3	110.9	183.4	326.9	326.9
Total new and used vehicle sales
Disaggregation of Revenue [Line Items]
Revenues	2,534.6	2,607.3	6,494.8		7,433.4
Total new and used vehicle sales | U.S.
Disaggregation of Revenue [Line Items]
Revenues	1,825.2	1,995.3	4,917.8		5,522.8
Total new and used vehicle sales | U.K.
Disaggregation of Revenue [Line Items]
Revenues	664.2	515.0	1,420.4		1,625.5
Total new and used vehicle sales | Brazil
Disaggregation of Revenue [Line Items]
Revenues	45.2	97.1	156.6		285.1
New vehicle retail sales
Disaggregation of Revenue [Line Items]
Revenues	1,580.7	1,652.3	3,985.5	4,632.2	4,632.2
New vehicle retail sales | U.S.
Disaggregation of Revenue [Line Items]
Revenues	1,172.2	1,291.8	3,076.3		3,512.3
New vehicle retail sales | U.K.
Disaggregation of Revenue [Line Items]
Revenues	376.6	290.7	800.1		911.5
New vehicle retail sales | Brazil
Disaggregation of Revenue [Line Items]
Revenues	31.9	69.9	109.1		208.4
Used vehicle retail sales
Disaggregation of Revenue [Line Items]
Revenues	867.2	869.7	2,287.4	2,527.8	2,527.8
Used vehicle retail sales | U.S.
Disaggregation of Revenue [Line Items]
Revenues	608.2	657.7	1,719.4		1,877.5
Used vehicle retail sales | U.K.
Disaggregation of Revenue [Line Items]
Revenues	248.1	189.3	529.7		586.8
Used vehicle retail sales | Brazil
Disaggregation of Revenue [Line Items]
Revenues	10.9	22.8	38.3		63.4
Used vehicle wholesale sales
Disaggregation of Revenue [Line Items]
Revenues	86.7	85.2	221.9	273.4	273.4
Used vehicle wholesale sales | U.S.
Disaggregation of Revenue [Line Items]
Revenues	44.8	45.8	122.1		132.9
Used vehicle wholesale sales | U.K.
Disaggregation of Revenue [Line Items]
Revenues	39.5	35.0	90.6		127.1
Used vehicle wholesale sales | Brazil
Disaggregation of Revenue [Line Items]
Revenues	2.4	4.4	9.2		13.3
Parts and service sales
Disaggregation of Revenue [Line Items]
Revenues	375.6	383.5	1,028.2	1,130.8	1,130.8
Parts and service sales | U.S.
Disaggregation of Revenue [Line Items]
Revenues	306.4	314.9	865.2		922.1
Parts and service sales | U.K.
Disaggregation of Revenue [Line Items]
Revenues	61.3	56.6	139.5		172.5
Parts and service sales | Brazil
Disaggregation of Revenue [Line Items]
Revenues	8.0	12.0	23.4		36.1
Finance, insurance and other, net
Disaggregation of Revenue [Line Items]
Revenues	129.5	127.5	338.7	368.2	368.2
Variable consideration recognized relating to performance obligations satisfied in previous period	7.6	2.6	16.9	$ 14.8
Finance, insurance and other, net | U.S.
Disaggregation of Revenue [Line Items]
Revenues	113.0	112.7	300.2		319.4
Finance, insurance and other, net | U.K.
Disaggregation of Revenue [Line Items]
Revenues	15.4	13.0	35.1		43.2
Finance, insurance and other, net | Brazil
Disaggregation of Revenue [Line Items]
Revenues	$ 1.1	$ 1.9	$ 3.4		$ 5.6