Revenue Recognition Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 6,379.5	$ 5,404.9	$ 19,261.7	$ 14,604.9
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	5,679.3	4,751.5	17,198.2	12,754.1
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	700.2	653.4	2,063.5	1,850.8
New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,753.8	2,748.4	9,164.4	7,291.6
Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,323.2	1,516.9	6,295.2	4,090.1
Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	943.7	852.8	2,745.5	2,419.0
Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	348.9	281.2	1,030.9	763.4
Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	9.9	5.6	25.7	40.8
AN Reportable Segment, Domestic [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,955.2	1,734.5	5,926.7	4,704.0
AN Reportable Segment, Domestic [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,778.6	1,563.4	5,404.3	4,218.0
AN Reportable Segment, Domestic [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	176.6	171.1	522.4	486.0
AN Reportable Segment, Domestic [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	791.3	923.4	2,718.0	2,424.0
AN Reportable Segment, Domestic [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	789.0	478.0	2,101.8	1,319.4
AN Reportable Segment, Domestic [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	257.2	233.2	749.2	662.1
AN Reportable Segment, Domestic [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	116.7	97.8	351.8	268.9
AN Reportable Segment, Domestic [Member] | Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	1.0	2.1	5.9	29.6
AN Reportable Segment, Import [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,983.3	1,621.5	5,927.9	4,308.9
AN Reportable Segment, Import [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,800.4	1,443.2	5,389.2	3,830.7
AN Reportable Segment, Import [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	182.9	178.3	538.7	478.2
AN Reportable Segment, Import [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	967.9	887.7	3,110.1	2,334.2
AN Reportable Segment, Import [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	638.0	415.0	1,727.2	1,105.3
AN Reportable Segment, Import [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	243.5	220.4	706.1	598.3
AN Reportable Segment, Import [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	127.1	95.2	369.4	261.6
AN Reportable Segment, Import [Member] | Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	6.8	3.2	15.1	9.5
AN Reportable Segment, Premium Luxury [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,218.0	1,870.0	6,790.0	5,051.6
AN Reportable Segment, Premium Luxury [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,960.6	1,633.6	6,028.7	4,387.4
AN Reportable Segment, Premium Luxury [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	257.4	236.4	761.3	664.2
AN Reportable Segment, Premium Luxury [Member] | New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	994.6	937.3	3,336.3	2,533.4
AN Reportable Segment, Premium Luxury [Member] | Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	810.9	576.7	2,242.0	1,534.3
AN Reportable Segment, Premium Luxury [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	312.4	278.4	915.2	779.0
AN Reportable Segment, Premium Luxury [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	98.7	77.5	293.5	204.8
AN Reportable Segment, Premium Luxury [Member] | Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	1.4	0.1	3.0	0.1
Corporate and Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	223.0	178.9	617.1	540.4
Corporate and Other [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	139.7	111.3	376.0	318.0
Corporate and Other [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	83.3	67.6	241.1	222.4
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	85.3	47.2	224.2	131.1
Corporate and Other [Member] | Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	130.6	120.8	375.0	379.6
Corporate and Other [Member] | Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	6.4	10.7	16.2	28.1
Corporate and Other [Member] | Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 0.7	$ 0.2	$ 1.7	$ 1.6