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Segment Information (Details) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segments	Jun. 30, 2017 USD ($) segments
Segment Reporting Information [Line Items]
Number of reportable segments | segments				3	3
Revenues		$ 5,392.0	$ 5,279.3	$ 10,651.9	$ 10,418.7
Segment income		230.1	219.3	450.9	432.9
AN Reportable Segment, Domestic [Member]
Segment Reporting Information [Line Items]
Revenues		1,826.1	1,844.7	3,599.8	3,645.5
Segment income	[1]	67.1	60.1	127.4	121.5
AN Reportable Segment, Import [Member]
Segment Reporting Information [Line Items]
Revenues		1,717.0	1,702.2	3,369.6	3,333.9
Segment income	[1]	78.1	75.3	150.9	147.0
AN Reportable Segment, Premium Luxury [Member]
Segment Reporting Information [Line Items]
Revenues		1,730.2	1,644.1	3,448.0	3,260.7
Segment income	[1]	84.9	83.9	172.6	164.4
Intercompany Revenue Eliminated [Member] | AN Reportable Segment, Domestic [Member]
Segment Reporting Information [Line Items]
Revenues		(133.6)	(139.9)	(274.2)	(278.7)
Intercompany Revenue Eliminated [Member] | AN Reportable Segment, Import [Member]
Segment Reporting Information [Line Items]
Revenues		(86.6)	(98.0)	(167.5)	(190.0)
Intercompany Revenue Eliminated [Member] | AN Reportable Segment, Premium Luxury [Member]
Segment Reporting Information [Line Items]
Revenues		$ (115.3)	$ (119.7)	$ (217.4)	$ (235.7)
[1] Segment income represents income for each of our reportable segments and is defined as operating income less floorplan interest expense.

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- Definition

Represents operating income less floorplan interest expense for each of the reportable segments.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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