Revenue Recognition Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues		$ 5,259.9	$ 5,139.4
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues		4,601.1
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	[1]	658.8
New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues		2,802.3	2,796.2
Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues		1,330.5	1,240.9
Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues		858.5	845.1
Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues		240.8	221.6
Other Goods and Services [Member]
Disaggregation of Revenue [Line Items]
Revenues		27.8	35.6
AN Reportable Segment, Domestic [Member]
Disaggregation of Revenue [Line Items]
Revenues		1,773.7	1,800.8
AN Reportable Segment, Domestic [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues		1,602.8
AN Reportable Segment, Domestic [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	[1]	170.9
AN Reportable Segment, Domestic [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues		933.6
AN Reportable Segment, Domestic [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues		462.8
AN Reportable Segment, Domestic [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues		270.5
AN Reportable Segment, Domestic [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues		85.8
AN Reportable Segment, Domestic [Member] | Other Goods and Services [Member]
Disaggregation of Revenue [Line Items]
Revenues		21.0
AN Reportable Segment, Import [Member]
Disaggregation of Revenue [Line Items]
Revenues		1,652.6	1,631.7
AN Reportable Segment, Import [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues		1,475.2
AN Reportable Segment, Import [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	[1]	177.4
AN Reportable Segment, Import [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues		955.8
AN Reportable Segment, Import [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues		366.8
AN Reportable Segment, Import [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues		234.3
AN Reportable Segment, Import [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues		89.3
AN Reportable Segment, Import [Member] | Other Goods and Services [Member]
Disaggregation of Revenue [Line Items]
Revenues		6.4
AN Reportable Segment, Premium Luxury [Member]
Disaggregation of Revenue [Line Items]
Revenues		1,717.8	$ 1,616.6
AN Reportable Segment, Premium Luxury [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues		1,492.2
AN Reportable Segment, Premium Luxury [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	[1]	225.6
AN Reportable Segment, Premium Luxury [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues		912.9
AN Reportable Segment, Premium Luxury [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues		478.0
AN Reportable Segment, Premium Luxury [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues		268.6
AN Reportable Segment, Premium Luxury [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues		58.0
AN Reportable Segment, Premium Luxury [Member] | Other Goods and Services [Member]
Disaggregation of Revenue [Line Items]
Revenues		0.3
Corporate and Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	[2]	115.8
Corporate and Other [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	[2]	30.9
Corporate and Other [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	[1],[2]	84.9
Corporate and Other [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	[2]	0.0
Corporate and Other [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	[2]	22.9
Corporate and Other [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	[2]	85.1
Corporate and Other [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	[2]	7.7
Corporate and Other [Member] | Other Goods and Services [Member]
Disaggregation of Revenue [Line Items]
Revenues	[2]	$ 0.1

[1] ) Represents revenue recognized during the period for automotive repair and maintenance services.

[2] “Corporate and other” is comprised of our other businesses, including collision centers, automotive auctions, and AutoNation USA stand-alone used vehicle sales and service centers.