Revenue Recognition Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 6,978.4	$ 4,533.0	$ 12,882.2	$ 9,200.0
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	6,262.4	4,001.2	11,518.9	8,002.6
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	716.0	531.8	1,363.3	1,197.4
New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	3,428.3	2,261.3	6,410.6	4,543.2
Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,222.9	1,324.5	3,972.0	2,573.2
Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	950.8	689.9	1,801.8	1,566.2
Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	369.0	246.4	682.0	482.2
Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	7.4	10.9	15.8	35.2
AN Reportable Segment, Domestic [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,124.8	1,486.0	3,971.5	2,969.5
AN Reportable Segment, Domestic [Member] \| Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,944.2	1,343.2	3,625.7	2,654.6
AN Reportable Segment, Domestic [Member] \| Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	180.6	142.8	345.8	314.9
AN Reportable Segment, Domestic [Member] \| New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	991.3	761.7	1,926.7	1,500.6
AN Reportable Segment, Domestic [Member] \| Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	749.1	436.8	1,312.8	841.4
AN Reportable Segment, Domestic [Member] \| Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	257.9	193.3	492.0	428.9
AN Reportable Segment, Domestic [Member] \| Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	125.8	88.0	235.1	171.1
AN Reportable Segment, Domestic [Member] \| Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.7	6.2	4.9	27.5
AN Reportable Segment, Import [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,175.0	1,325.3	3,944.6	2,687.4
AN Reportable Segment, Import [Member] \| Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,984.7	1,187.0	3,588.8	2,387.5
AN Reportable Segment, Import [Member] \| Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	190.3	138.3	355.8	299.9
AN Reportable Segment, Import [Member] \| New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,184.2	712.6	2,142.2	1,446.5
AN Reportable Segment, Import [Member] \| Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	604.5	355.8	1,089.2	690.3
AN Reportable Segment, Import [Member] \| Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	248.5	170.4	462.6	377.9
AN Reportable Segment, Import [Member] \| Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	133.2	82.4	242.3	166.4
AN Reportable Segment, Import [Member] \| Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	4.6	4.1	8.3	6.3
AN Reportable Segment, Premium Luxury [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,468.5	1,564.8	4,572.0	3,181.6
AN Reportable Segment, Premium Luxury [Member] \| Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,204.5	1,373.6	4,068.1	2,753.8
AN Reportable Segment, Premium Luxury [Member] \| Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	264.0	191.2	503.9	427.8
AN Reportable Segment, Premium Luxury [Member] \| New Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,252.8	787.0	2,341.7	1,596.1
AN Reportable Segment, Premium Luxury [Member] \| Used Vehicle [Member]
Disaggregation of Revenue [Line Items]
Revenues	788.8	490.3	1,431.1	957.6
AN Reportable Segment, Premium Luxury [Member] \| Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	317.6	223.2	602.8	500.6
AN Reportable Segment, Premium Luxury [Member] \| Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	108.0	64.3	194.8	127.3
AN Reportable Segment, Premium Luxury [Member] \| Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	1.3	0.0	1.6	0.0
Corporate and Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	210.1	156.9	394.1	361.5
Corporate and Other [Member] \| Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	129.0	97.4	236.3	206.7
Corporate and Other [Member] \| Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenues	81.1	59.5	157.8	154.8
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	80.5	41.6	138.9	83.9
Corporate and Other [Member] \| Parts and Service [Member]
Disaggregation of Revenue [Line Items]
Revenues	126.8	103.0	244.4	258.8
Corporate and Other [Member] \| Finance and Insurance, Net [Member]
Disaggregation of Revenue [Line Items]
Revenues	2.0	11.7	9.8	17.4
Corporate and Other [Member] \| Product and Service, Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 0.8	$ 0.6	$ 1.0	$ 1.4