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Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Jun. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Changes in Goodwill
Balance at the beginning of the year		$ 1,271.2
Foreign currency		(1.3)
Balance at the end of the year		1,269.9
Deferred income tax expense (benefit)		(0.1)	$ (26.8)
Impairment of goodwill		0.0
U.S. Dealer-to-Dealer
Changes in Goodwill
Impairment of goodwill	$ 218.9
Europe
Changes in Goodwill
Impairment of goodwill	6.4
Domestic Tax Jurisdiction
Changes in Goodwill
Deferred Tax Assets, Valuation Allowance		42.9		$ 36.4
Goodwill
Changes in Goodwill
Deferred income tax expense (benefit)	(52.5)
ADESA Trade Names (Indefinite)
Changes in Goodwill
Deferred income tax expense (benefit)	(6.5)
ADESA tradename - non-cash impairment charge	$ 25.5
Marketplace
Changes in Goodwill
Balance at the beginning of the year		1,030.3
Foreign currency		(1.3)
Balance at the end of the year		1,029.0
Accumulated goodwill impairment		250.8		250.8
Finance
Changes in Goodwill
Balance at the beginning of the year		240.9
Foreign currency		0.0
Balance at the end of the year		240.9
Accumulated goodwill impairment		$ 161.5		$ 161.5