Condensed Consolidating Financial Information - Balance Sheet (Details) - USD ($) $ in Millions	Jun. 30, 2018	Jan. 02, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Condensed consolidating balance sheet
Cash and cash equivalents	$ 45.8		$ 45.7	$ 20.7	$ 24.0
Accounts receivable, net	919.6	$ 977.3	954.9
Inventories	3,895.1	3,930.7	3,944.1
Other current assets	97.5		81.8
Total current assets	4,958.0		5,026.5
Property and equipment, net	2,144.9		2,108.6
Intangible assets	2,174.8		2,134.5
Equity method investments	1,279.2		1,256.6
Other long-term assets	13.6		14.4
Total assets	10,570.5		10,540.6
Floor plan notes payable	2,278.0		2,343.2
Floor plan notes payable - non-trade	1,325.7		1,418.6
Accounts payable	680.0		641.6
Accrued expenses	529.7	523.6	523.5
Current portion of long-term debt	92.3		72.8
Liabilities held for sale	0.7		0.7
Total current liabilities	4,906.4		5,000.4
Long-term debt	2,059.6		2,090.4
Deferred tax liabilities	530.8	$ 483.8	481.5
Other long-term liabilities	547.9		540.3
Total liabilities	8,044.7		8,112.6
Total equity	2,525.8		2,428.0
Total liabilities and equity	$ 10,570.5		$ 10,540.6
3.75% senior subordinated notes due 2020
Condensed consolidating balance sheet
Interest rate (as a percent)	3.75%		3.75%
Domestic Subsidiaries ownership guaranteeing obligations (as a percent)	100.00%
5.75% senior subordinated notes due 2022
Condensed consolidating balance sheet
Interest rate (as a percent)	5.75%		5.75%
Domestic Subsidiaries ownership guaranteeing obligations (as a percent)	100.00%
5.375% senior subordinated notes due 2024
Condensed consolidating balance sheet
Interest rate (as a percent)	5.375%		5.375%
Domestic Subsidiaries ownership guaranteeing obligations (as a percent)	100.00%
5.50% senior subordinated notes due 2026
Condensed consolidating balance sheet
Interest rate (as a percent)	5.50%		5.50%
Domestic Subsidiaries ownership guaranteeing obligations (as a percent)	100.00%
Eliminations
Condensed consolidating balance sheet
Accounts receivable, net	$ (472.2)		$ (463.6)
Total current assets	(472.2)		(463.6)
Other long-term assets	(2,784.9)		(2,772.7)
Total assets	(3,257.1)		(3,236.3)
Accrued expenses	(472.2)		(463.6)
Total current liabilities	(472.2)		(463.6)
Long-term debt	(148.0)		(150.2)
Total liabilities	(620.2)		(613.8)
Total equity	(2,636.9)		(2,622.5)
Total liabilities and equity	(3,257.1)		(3,236.3)
Reportable legal entities | Penske Automotive Group
Condensed consolidating balance sheet
Accounts receivable, net	472.2		463.6
Other current assets	6.0		7.3
Total current assets	478.2		470.9
Property and equipment, net	2.7		3.3
Equity method investments	1,209.9		1,186.9
Other long-term assets	2,789.8		2,777.8
Total assets	4,480.6		4,438.9
Floor plan notes payable - non-trade	198.4		196.6
Accounts payable	4.2		3.9
Accrued expenses	1.1		1.0
Total current liabilities	203.7		201.5
Long-term debt	1,751.1		1,809.4
Total liabilities	1,954.8		2,010.9
Total equity	2,525.8		2,428.0
Total liabilities and equity	4,480.6		4,438.9
Reportable legal entities | Guarantor Subsidiaries
Condensed consolidating balance sheet
Cash and cash equivalents	0.4		14.8	1.9	9.4
Accounts receivable, net	421.5		523.8
Inventories	1,893.0		1,935.0
Other current assets	20.4		16.8
Total current assets	2,335.3		2,490.4
Property and equipment, net	1,019.3		1,032.9
Intangible assets	1,328.7		1,334.6
Other long-term assets	4.3		4.6
Total assets	4,687.6		4,862.5
Floor plan notes payable	1,221.5		1,272.4
Floor plan notes payable - non-trade	503.2		601.9
Accounts payable	163.5		194.4
Accrued expenses	163.9		165.2
Current portion of long-term debt	5.0		5.5
Liabilities held for sale	0.7		0.7
Total current liabilities	2,057.8		2,240.1
Long-term debt	191.2		191.6
Deferred tax liabilities	529.8		480.1
Other long-term liabilities	64.4		64.7
Total liabilities	2,843.2		2,976.5
Total equity	1,844.4		1,886.0
Total liabilities and equity	4,687.6		4,862.5
Reportable legal entities | Non-Guarantor Subsidiaries
Condensed consolidating balance sheet
Cash and cash equivalents	45.4		30.9	$ 18.8	$ 14.6
Accounts receivable, net	498.1		431.1
Inventories	2,002.1		2,009.1
Other current assets	71.1		57.7
Total current assets	2,616.7		2,528.8
Property and equipment, net	1,122.9		1,072.4
Intangible assets	846.1		799.9
Equity method investments	69.3		69.7
Other long-term assets	4.4		4.7
Total assets	4,659.4		4,475.5
Floor plan notes payable	1,056.5		1,070.8
Floor plan notes payable - non-trade	624.1		620.1
Accounts payable	512.3		443.3
Accrued expenses	836.9		820.9
Current portion of long-term debt	87.3		67.3
Total current liabilities	3,117.1		3,022.4
Long-term debt	265.3		239.6
Deferred tax liabilities	1.0		1.4
Other long-term liabilities	483.5		475.6
Total liabilities	3,866.9		3,739.0
Total equity	792.5		736.5
Total liabilities and equity	$ 4,659.4		$ 4,475.5