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Revenues - Revenues Disaggregated by Revenue Source and Geographical Segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 3,111.4	$ 3,118.3	$ 3,005.7	$ 2,808.4	$ 2,908.8	$ 2,889.1	$ 2,943.5	$ 2,860.0	$ 12,043.8	$ 11,601.4	$ 11,123.7
Variable consideration recognized									19.5	18.7
U.S.
Disaggregation of Revenue [Line Items]
Revenues									9,184.2	8,723.3	8,680.6
U.K.
Disaggregation of Revenue [Line Items]
Revenues									2,413.7	2,437.4	1,986.0
Brazil
Disaggregation of Revenue [Line Items]
Revenues									445.9	440.7	457.2
Total new and used vehicle sales
Disaggregation of Revenue [Line Items]
Revenues									10,035.9	9,717.0	9,356.7
Total new and used vehicle sales \| U.S.
Disaggregation of Revenue [Line Items]
Revenues									7,516.6	7,168.7	7,180.2
Total new and used vehicle sales \| U.K.
Disaggregation of Revenue [Line Items]
Revenues									2,128.7	2,162.6	1,775.7
Total new and used vehicle sales \| Brazil
Disaggregation of Revenue [Line Items]
Revenues									390.6	385.7	400.7
New vehicle retail sales
Disaggregation of Revenue [Line Items]
Revenues									6,314.1	6,181.4	6,157.5
New vehicle retail sales \| U.S.
Disaggregation of Revenue [Line Items]
Revenues									4,832.2	4,682.8	4,768.9
New vehicle retail sales \| U.K.
Disaggregation of Revenue [Line Items]
Revenues									1,195.1	1,217.1	1,092.6
New vehicle retail sales \| Brazil
Disaggregation of Revenue [Line Items]
Revenues									286.8	281.4	296.1
Used vehicle retail sales
Disaggregation of Revenue [Line Items]
Revenues									3,366.6	3,166.1	2,799.0
Used vehicle retail sales \| U.S.
Disaggregation of Revenue [Line Items]
Revenues									2,509.9	2,307.0	2,160.7
Used vehicle retail sales \| U.K.
Disaggregation of Revenue [Line Items]
Revenues									771.3	771.7	546.3
Used vehicle retail sales \| Brazil
Disaggregation of Revenue [Line Items]
Revenues									85.4	87.4	92.0
Used vehicle wholesale sales
Disaggregation of Revenue [Line Items]
Revenues									355.2	369.6	400.2
Used vehicle wholesale sales \| U.S.
Disaggregation of Revenue [Line Items]
Revenues									174.5	178.9	250.7
Used vehicle wholesale sales \| U.K.
Disaggregation of Revenue [Line Items]
Revenues									162.3	173.8	136.8
Used vehicle wholesale sales \| Brazil
Disaggregation of Revenue [Line Items]
Revenues									18.3	16.9	12.7
Parts and service sales
Disaggregation of Revenue [Line Items]
Revenues									1,510.0	1,416.9	1,338.0
Parts and service sales \| U.S.
Disaggregation of Revenue [Line Items]
Revenues									1,234.4	1,153.3	1,124.4
Parts and service sales \| U.K.
Disaggregation of Revenue [Line Items]
Revenues									227.9	217.6	165.8
Parts and service sales \| Brazil
Disaggregation of Revenue [Line Items]
Revenues									47.6	46.0	47.9
Finance, insurance and other, net
Disaggregation of Revenue [Line Items]
Revenues									497.9	467.5	429.0
Finance, insurance and other, net \| U.S.
Disaggregation of Revenue [Line Items]
Revenues									433.2	401.3	376.0
Finance, insurance and other, net \| U.K.
Disaggregation of Revenue [Line Items]
Revenues									57.0	57.2	44.5
Finance, insurance and other, net \| Brazil
Disaggregation of Revenue [Line Items]
Revenues									$ 7.6	$ 9.0	$ 8.5