Interim Financial Statements - Recent Accounting Pronouncements - ASU 2014-09 - Initial Application (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 02, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Accounts receivable, net	$ 919.6		$ 919.6		$ 977.3		$ 954.9
Inventories	3,895.1		3,895.1		3,930.7		3,944.1
Liabilities and Equity
Accrued expenses	529.7		529.7		523.6		523.5
Deferred tax liabilities	530.8		530.8		483.8		481.5
Retained earnings	2,199.7		2,199.7		$ 2,016.0		$ 2,009.4
Statement of Income
Gross profit	889.8	$ 817.3	1,754.2	$ 1,591.6
Income taxes	(41.0)	(50.2)	(77.6)	(91.3)
Net income	135.2	106.9	243.0	189.9
ASC 606 | Without Adoption of ASC 606
Assets
Accounts receivable, net	890.6		890.6
Inventories	3,912.0		3,912.0
Liabilities and Equity
Accrued expenses	529.5		529.5
Deferred tax liabilities	527.7		527.7
Retained earnings	2,190.9		2,190.9
Statement of Income
Gross profit	887.4		1,751.2
Income taxes	(40.3)		(76.7)
Net income	133.5		240.9
ASC 606 | Adjustment Impact of Adoption of ASC 606
Assets
Accounts receivable, net	29.0		29.0			$ 22.4
Inventories	(16.9)		(16.9)			(13.4)
Liabilities and Equity
Accrued expenses	0.2		0.2			0.1
Deferred tax liabilities	3.1		3.1			2.3
Retained earnings	8.8		8.8			$ 6.6
Statement of Income
Gross profit	2.4		3.0
Income taxes	0.7		0.9
Net income	1.7		2.1
Retail Automotive Dealership
Statement of Income
Revenues	5,455.5		10,751.5
Cost of sales	4,657.5		9,175.2
Retail Automotive Dealership | Without Adoption of ASC 606
Statement of Income
Revenues		5,040.7		9,797.1
Retail Automotive Dealership | ASC 606 | Without Adoption of ASC 606
Statement of Income
Revenues	5,453.3		10,748.8
Cost of sales	4,656.3		9,173.4
Retail Automotive Dealership | ASC 606 | Adjustment Impact of Adoption of ASC 606
Statement of Income
Revenues	2.2		2.7
Cost of sales	1.2		1.8
Retail Commercial Truck Dealership
Statement of Income
Revenues	338.8		631.2
Cost of sales	286.4		532.2
Retail Commercial Truck Dealership | Without Adoption of ASC 606
Statement of Income
Revenues		228.5		440.2
Retail Commercial Truck Dealership | ASC 606 | Without Adoption of ASC 606
Statement of Income
Revenues	339.6		631.1
Cost of sales	286.8		532.1
Retail Commercial Truck Dealership | ASC 606 | Adjustment Impact of Adoption of ASC 606
Statement of Income
Revenues	(0.8)		0.1
Cost of sales	(0.4)		0.1
Commercial Vehicle Distribution and Other
Statement of Income
Revenues	146.0		304.5
Cost of sales	106.6		225.6
Commercial Vehicle Distribution and Other | Without Adoption of ASC 606
Statement of Income
Revenues		$ 114.2		$ 227.2
Commercial Vehicle Distribution and Other | ASC 606 | Without Adoption of ASC 606
Statement of Income
Revenues	143.7		300.7
Cost of sales	106.1		223.9
Commercial Vehicle Distribution and Other | ASC 606 | Adjustment Impact of Adoption of ASC 606
Statement of Income
Revenues	2.3		3.8
Cost of sales	$ 0.5		$ 1.7