Organization and Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 02, 2018	Jan. 01, 2018
Assets
Accounts receivable, net	$ 929.1				$ 954.9				$ 929.1	$ 954.9		$ 977.3
Inventories	4,040.1				3,944.1				4,040.1	3,944.1		3,930.7
Liabilities and Equity
Accrued expenses	566.6				523.5				566.6	523.5		523.6
Deferred tax liabilities	577.8				481.5				577.8	481.5		483.8
Retained earnings	2,365.8				2,009.4				2,365.8	2,009.4		$ 2,016.0
Statement of Income
Revenues	5,439.3	$ 5,658.6	$ 5,940.3	$ 5,746.9	5,398.0	$ 5,524.4	$ 5,383.4	$ 5,081.1	22,785.1	21,386.9	$ 20,118.5
Cost of sales									19,370.2	18,164.4	17,151.9
Gross profit	808.1	852.6	889.8	864.4	808.3	822.6	817.3	774.3	3,414.9	3,222.5	2,966.6
Income taxes									(134.3)	64.8	(160.7)
Net income	97.2	$ 130.1	$ 135.2	$ 107.8	$ 328.6	$ 94.3	$ 106.9	$ 83.0	470.3	612.8	346.4
ASC 606 | Without Adoption of ASC 606
Assets
Accounts receivable, net	899.9								899.9
Inventories	4,057.2								4,057.2
Liabilities and Equity
Accrued expenses	566.2								566.2
Deferred tax liabilities	574.8								574.8
Retained earnings	2,357.1								2,357.1
Statement of Income
Gross profit									3,412.1
Income taxes									(133.5)
Net income									468.3
ASC 606 | Adjustment Impact of Adoption of ASC 606
Assets
Accounts receivable, net	29.2								29.2				$ 22.4
Inventories	(17.1)								(17.1)				(13.4)
Liabilities and Equity
Accrued expenses	0.4								0.4				0.1
Deferred tax liabilities	3.0								3.0				2.3
Retained earnings	$ 8.7								8.7				$ 6.6
Statement of Income
Gross profit									2.8
Income taxes									0.8
Net income									2.0
Retail Automotive Dealership
Statement of Income
Revenues									20,849.2	19,824.3	18,673.2
Cost of sales									17,790.6	16,899.5	15,968.4
Retail Automotive Dealership | Without Adoption of ASC 606
Statement of Income
Revenues										19,824.3	18,673.2
Retail Automotive Dealership | ASC 606 | Without Adoption of ASC 606
Statement of Income
Revenues									20,846.0
Cost of sales									17,788.5
Retail Automotive Dealership | ASC 606 | Adjustment Impact of Adoption of ASC 606
Statement of Income
Revenues									3.2
Cost of sales									2.1
Retail Commercial Truck Dealership
Statement of Income
Revenues									1,374.5	1,048.0	1,000.7
Cost of sales									1,163.0	882.2	857.8
Retail Commercial Truck Dealership | Without Adoption of ASC 606
Statement of Income
Revenues										1,048.0	1,000.7
Retail Commercial Truck Dealership | ASC 606 | Without Adoption of ASC 606
Statement of Income
Revenues									1,373.9
Cost of sales									1,162.7
Retail Commercial Truck Dealership | ASC 606 | Adjustment Impact of Adoption of ASC 606
Statement of Income
Revenues									0.6
Cost of sales									0.3
Commercial Vehicle Distribution and Other
Statement of Income
Revenues									561.4	514.6	444.6
Cost of sales									416.6	382.7	325.7
Commercial Vehicle Distribution and Other | Without Adoption of ASC 606
Statement of Income
Revenues										$ 514.6	$ 444.6
Commercial Vehicle Distribution and Other | ASC 606 | Without Adoption of ASC 606
Statement of Income
Revenues									558.5
Cost of sales									415.1
Commercial Vehicle Distribution and Other | ASC 606 | Adjustment Impact of Adoption of ASC 606
Statement of Income
Revenues									2.9
Cost of sales									$ 1.5