Interim Financial Statements - Recent Accounting Pronouncements - ASU 2014-09 - Initial Application (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 02, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Accounts receivable, net	$ 1,035.4		$ 977.3		$ 954.9
Inventories	3,972.2		3,930.7		3,944.1
Liabilities and Equity
Accrued expenses	563.9		523.6		523.5
Deferred tax liabilities	505.5		483.8		481.5
Retained earnings	2,094.9		$ 2,016.0		$ 2,009.4
Statement of Income
Gross profit	864.4	$ 774.3
Income taxes	(36.6)	(41.1)
Net income	107.8	83.0
ASC 606 | Without Adoption of ASC 606
Assets
Accounts receivable, net	1,010.1
Inventories	3,987.9
Liabilities and Equity
Accrued expenses	563.8
Deferred tax liabilities	503.0
Retained earnings	2,087.9
Statement of Income
Gross profit	863.8
Income taxes	(36.4)
Net income	107.4
ASC 606 | Adjustment Impact of Adoption of ASC 606
Assets
Accounts receivable, net	25.3			$ 22.4
Inventories	(15.7)			(13.4)
Liabilities and Equity
Accrued expenses	0.1			0.1
Deferred tax liabilities	2.5			2.3
Retained earnings	7.0			$ 6.6
Statement of Income
Gross profit	0.6
Income taxes	0.2
Net income	0.4
Retail Automotive Dealership
Statement of Income
Revenues	5,296.0
Cost of sales	4,517.7
Retail Automotive Dealership | Without Adoption of ASC 606
Statement of Income
Revenues		4,756.4
Retail Automotive Dealership | ASC 606 | Without Adoption of ASC 606
Statement of Income
Revenues	5,295.5
Cost of sales	4,517.1
Retail Automotive Dealership | ASC 606 | Adjustment Impact of Adoption of ASC 606
Statement of Income
Revenues	0.5
Cost of sales	0.6
Retail Commercial Truck Dealership
Statement of Income
Revenues	292.4
Cost of sales	245.8
Retail Commercial Truck Dealership | Without Adoption of ASC 606
Statement of Income
Revenues		211.7
Retail Commercial Truck Dealership | ASC 606 | Without Adoption of ASC 606
Statement of Income
Revenues	291.5
Cost of sales	245.3
Retail Commercial Truck Dealership | ASC 606 | Adjustment Impact of Adoption of ASC 606
Statement of Income
Revenues	0.9
Cost of sales	0.5
Commercial Vehicle Distribution and Other
Statement of Income
Revenues	158.5
Cost of sales	119.0
Commercial Vehicle Distribution and Other | Without Adoption of ASC 606
Statement of Income
Revenues		$ 113.0
Commercial Vehicle Distribution and Other | ASC 606 | Without Adoption of ASC 606
Statement of Income
Revenues	157.0
Cost of sales	117.8
Commercial Vehicle Distribution and Other | ASC 606 | Adjustment Impact of Adoption of ASC 606
Statement of Income
Revenues	1.5
Cost of sales	$ 1.2