Inventory and Vehicle Floorplan Payable (Components of Inventory) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Inventory [Line Items]
Inventory	$ 2,598.5	$ 3,305.8
New Vehicle [Member]
Inventory [Line Items]
Inventory	1,761.9	2,490.8
Used Vehicle [Member]
Inventory [Line Items]
Inventory	648.4	570.0
Parts and Service [Member]
Inventory [Line Items]
Inventory	$ 188.2	$ 245.0

Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121566466&loc=d3e6676-107765

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.6(a))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Name:	us-gaap_InventoryNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant