Revenue Recognition Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 5,404.9	$ 5,461.2	$ 14,604.9	$ 15,786.8
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	4,751.5	4,768.6	12,754.1	13,731.7
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	653.4	692.6	1,850.8	2,055.1
New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,748.4	2,874.8	7,291.6	8,141.1
Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,516.9	1,402.9	4,090.1	4,121.9
Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	852.8	902.6	2,419.0	2,680.8
Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	281.2	266.2	763.4	757.9
Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	5.6	14.7	40.8	85.1
AN Reportable Segment, Domestic [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,734.5	1,728.6	4,704.0	5,004.7
AN Reportable Segment, Domestic [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,563.4	1,547.1	4,218.0	4,473.6
AN Reportable Segment, Domestic [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	171.1	181.5	486.0	531.1
AN Reportable Segment, Domestic [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	923.4	925.4	2,424.0	2,600.9
AN Reportable Segment, Domestic [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	478.0	456.6	1,319.4	1,347.0
AN Reportable Segment, Domestic [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	233.2	244.7	662.1	722.2
AN Reportable Segment, Domestic [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	97.8	95.1	268.9	265.3
AN Reportable Segment, Domestic [Member] | Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	2.1	6.8	29.6	69.3
AN Reportable Segment, Import [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,621.5	1,722.5	4,308.9	4,845.9
AN Reportable Segment, Import [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,443.2	1,543.1	3,830.7	4,313.3
AN Reportable Segment, Import [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	178.3	179.4	478.2	532.6
AN Reportable Segment, Import [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	887.7	1,003.8	2,334.2	2,740.1
AN Reportable Segment, Import [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	415.0	386.4	1,105.3	1,143.9
AN Reportable Segment, Import [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	220.4	226.1	598.3	675.5
AN Reportable Segment, Import [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	95.2	99.9	261.6	275.8
AN Reportable Segment, Import [Member] | Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	3.2	6.3	9.5	10.6
AN Reportable Segment, Premium Luxury [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,870.0	1,820.5	5,051.6	5,375.5
AN Reportable Segment, Premium Luxury [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,633.6	1,579.0	4,387.4	4,654.0
AN Reportable Segment, Premium Luxury [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	236.4	241.5	664.2	721.5
AN Reportable Segment, Premium Luxury [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	937.3	945.6	2,533.4	2,800.1
AN Reportable Segment, Premium Luxury [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	576.7	519.7	1,534.3	1,524.2
AN Reportable Segment, Premium Luxury [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	278.4	283.8	779.0	847.7
AN Reportable Segment, Premium Luxury [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	77.5	70.0	204.8	200.0
AN Reportable Segment, Premium Luxury [Member] | Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.1	1.4	0.1	3.5
Corporate and Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	178.9	189.6	540.4	560.7
Corporate and Other [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	111.3	99.4	318.0	290.8
Corporate and Other [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	67.6	90.2	222.4	269.9
Corporate and Other [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Corporate and Other [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	47.2	40.2	131.1	106.8
Corporate and Other [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	120.8	148.0	379.6	435.4
Corporate and Other [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	10.7	1.2	28.1	16.8
Corporate and Other [Member] | Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 0.2	$ 0.2	$ 1.6	$ 1.7