Interim Financial Statements - Recent Accounting Pronouncements - ASU 2014-09 - Initial Application (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 02, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Accounts receivable, net	$ 953.4		$ 953.4		$ 977.3		$ 954.9
Inventories	3,783.4		3,783.4		3,930.7		3,944.1
Liabilities and Equity
Accrued expenses	570.2		570.2		523.6		523.5
Deferred tax liabilities	547.9		547.9		483.8		481.5
Retained earnings	2,299.2		2,299.2		$ 2,016.0		$ 2,009.4
Statement of Income
Gross profit	852.6	$ 822.6	2,606.8	$ 2,414.2
Income taxes	(27.1)	(44.7)	(104.7)	(136.0)
Net income	130.1	94.3	373.1	284.2
ASC 606 | Without Adoption of ASC 606
Assets
Accounts receivable, net	925.7		925.7
Inventories	3,799.2		3,799.2
Liabilities and Equity
Accrued expenses	569.8		569.8
Deferred tax liabilities	544.9		544.9
Retained earnings	2,290.7		2,290.7
Statement of Income
Gross profit	853.0		2,604.2
Income taxes	(27.2)		(103.9)
Net income	130.4		371.3
ASC 606 | Adjustment Impact of Adoption of ASC 606
Assets
Accounts receivable, net	27.7		27.7			$ 22.4
Inventories	(15.8)		(15.8)			(13.4)
Liabilities and Equity
Accrued expenses	0.4		0.4			0.1
Deferred tax liabilities	3.0		3.0			2.3
Retained earnings	8.5		8.5			$ 6.6
Statement of Income
Gross profit	(0.4)		2.6
Income taxes	(0.1)		0.8
Net income	(0.3)		1.8
Retail Automotive Dealership
Statement of Income
Revenues	5,148.5		15,900.0
Cost of sales	4,386.3		13,561.5
Retail Automotive Dealership | Without Adoption of ASC 606
Statement of Income
Revenues		5,085.4		14,882.5
Retail Automotive Dealership | ASC 606 | Without Adoption of ASC 606
Statement of Income
Revenues	5,149.4		15,898.2
Cost of sales	4,387.0		13,560.4
Retail Automotive Dealership | ASC 606 | Adjustment Impact of Adoption of ASC 606
Statement of Income
Revenues	(0.9)		1.8
Cost of sales	(0.7)		1.1
Retail Commercial Truck Dealership
Statement of Income
Revenues	385.3		1,016.5
Cost of sales	327.5		859.7
Retail Commercial Truck Dealership | Without Adoption of ASC 606
Statement of Income
Revenues		299.6		739.8
Retail Commercial Truck Dealership | ASC 606 | Without Adoption of ASC 606
Statement of Income
Revenues	384.3		1,015.4
Cost of sales	327.0		859.1
Retail Commercial Truck Dealership | ASC 606 | Adjustment Impact of Adoption of ASC 606
Statement of Income
Revenues	1.0		1.1
Cost of sales	0.5		0.6
Commercial Vehicle Distribution and Other
Statement of Income
Revenues	124.8		429.3
Cost of sales	92.2		317.8
Commercial Vehicle Distribution and Other | Without Adoption of ASC 606
Statement of Income
Revenues		$ 139.4		$ 366.6
Commercial Vehicle Distribution and Other | ASC 606 | Without Adoption of ASC 606
Statement of Income
Revenues	126.2		426.9
Cost of sales	92.9		316.8
Commercial Vehicle Distribution and Other | ASC 606 | Adjustment Impact of Adoption of ASC 606
Statement of Income
Revenues	(1.4)		2.4
Cost of sales	$ (0.7)		$ 1.0