Revenue Recognition Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 5,903.8	$ 4,667.0
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	5,256.6	4,001.5
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	647.2	665.5
New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,982.3	2,281.9
Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,749.1	1,248.7
Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	851.0	876.3
Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	313.0	235.8
Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	8.4	24.3
AN Reportable Segment, Domestic [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,846.7	1,483.5
AN Reportable Segment, Domestic [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,681.6	1,311.4
AN Reportable Segment, Domestic [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	165.1	172.1
AN Reportable Segment, Domestic [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	935.4	738.9
AN Reportable Segment, Domestic [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	563.7	404.6
AN Reportable Segment, Domestic [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	234.1	235.6
AN Reportable Segment, Domestic [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	109.3	83.1
AN Reportable Segment, Domestic [Member] | Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	4.2	21.3
AN Reportable Segment, Import [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,769.6	1,362.1
AN Reportable Segment, Import [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,604.1	1,200.5
AN Reportable Segment, Import [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	165.5	161.6
AN Reportable Segment, Import [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	958.0	733.9
AN Reportable Segment, Import [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	484.7	334.5
AN Reportable Segment, Import [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	214.1	207.5
AN Reportable Segment, Import [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	109.1	84.0
AN Reportable Segment, Import [Member] | Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	3.7	2.2
AN Reportable Segment, Premium Luxury [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,103.5	1,616.8
AN Reportable Segment, Premium Luxury [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,863.6	1,380.2
AN Reportable Segment, Premium Luxury [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	239.9	236.6
AN Reportable Segment, Premium Luxury [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,088.9	809.1
AN Reportable Segment, Premium Luxury [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	642.3	467.3
AN Reportable Segment, Premium Luxury [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	285.2	277.4
AN Reportable Segment, Premium Luxury [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	86.8	63.0
AN Reportable Segment, Premium Luxury [Member] | Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.3	0.0
Corporate and Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	184.0	204.6
Corporate and Other [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	107.3	109.4
Corporate and Other [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	76.7	95.2
Corporate and Other [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0
Corporate and Other [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	58.4	42.3
Corporate and Other [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	117.6	155.8
Corporate and Other [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	7.8	5.7
Corporate and Other [Member] | Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 0.2	$ 0.8