Inventory and Vehicle Floorplan Payable (Components Of Inventory) (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Inventory [Line Items]
Inventory	$ 3,489.2	$ 3,360.0
New vehicle
Inventory [Line Items]
Inventory	2,382.6	2,341.4
Used vehicle
Inventory [Line Items]
Inventory	847.8	754.1
Parts and service
Inventory [Line Items]
Inventory	$ 258.8	$ 264.5

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://www.xbrl.org/2003/role/exampleRef
-Topic 852
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481372/852-10-55-10

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483467/210-10-45-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(6))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant