Condensed Consolidating Financial Information - Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jan. 02, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Condensed consolidating balance sheet
Cash and cash equivalents	$ 37.6		$ 45.7	$ 37.2	$ 24.0
Accounts receivable, net	953.4	$ 977.3	954.9
Inventories	3,783.4	3,930.7	3,944.1
Other current assets	91.8		81.8
Total current assets	4,866.2		5,026.5
Property and equipment, net	2,178.2		2,108.6
Intangible assets	2,164.5		2,134.5
Equity method investments	1,299.7		1,256.6
Other long-term assets	18.3		14.4
Total assets	10,526.9		10,540.6
Floor plan notes payable	2,169.5		2,343.2
Floor plan notes payable - non-trade	1,359.0		1,418.6
Accounts payable	633.8		641.6
Accrued expenses	570.2	523.6	523.5
Current portion of long-term debt	92.7		72.8
Liabilities held for sale	0.7		0.7
Total current liabilities	4,825.9		5,000.4
Long-term debt	1,993.3		2,090.4
Deferred tax liabilities	547.9	$ 483.8	481.5
Other long-term liabilities	547.8		540.3
Total liabilities	7,914.9		8,112.6
Total equity	2,612.0		2,428.0
Total liabilities and equity	$ 10,526.9		$ 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	$ (477.0)		$ (463.6)
Total current assets	(477.0)		(463.6)
Other long-term assets	(2,719.5)		(2,772.7)
Total assets	(3,196.5)		(3,236.3)
Accrued expenses	(477.0)		(463.6)
Total current liabilities	(477.0)		(463.6)
Long-term debt	(92.5)		(150.2)
Total liabilities	(569.5)		(613.8)
Total equity	(2,627.0)		(2,622.5)
Total liabilities and equity	(3,196.5)		(3,236.3)
Reportable legal entities | Penske Automotive Group
Condensed consolidating balance sheet
Cash and cash equivalents	0.0
Accounts receivable, net	477.0		463.6
Other current assets	5.0		7.3
Total current assets	482.0		470.9
Property and equipment, net	2.7		3.3
Equity method investments	1,235.3		1,186.9
Other long-term assets	2,724.9		2,777.8
Total assets	4,444.9		4,438.9
Floor plan notes payable - non-trade	173.3		196.6
Accounts payable	5.5		3.9
Accrued expenses	1.2		1.0
Total current liabilities	180.0		201.5
Long-term debt	1,652.9		1,809.4
Total liabilities	1,832.9		2,010.9
Total equity	2,612.0		2,428.0
Total liabilities and equity	4,444.9		4,438.9
Reportable legal entities | Guarantor Subsidiaries
Condensed consolidating balance sheet
Cash and cash equivalents	3.4		14.8		9.4
Accounts receivable, net	435.1		523.8
Inventories	1,897.7		1,935.0
Other current assets	17.2		16.8
Total current assets	2,353.4		2,490.4
Property and equipment, net	1,034.5		1,032.9
Intangible assets	1,328.7		1,334.6
Other long-term assets	4.2		4.6
Total assets	4,720.8		4,862.5
Floor plan notes payable	1,193.7		1,272.4
Floor plan notes payable - non-trade	551.8		601.9
Accounts payable	173.7		194.4
Accrued expenses	187.1		165.2
Current portion of long-term debt	5.0		5.5
Liabilities held for sale	0.7		0.7
Total current liabilities	2,112.0		2,240.1
Long-term debt	189.0		191.6
Deferred tax liabilities	545.0		480.1
Other long-term liabilities	64.5		64.7
Total liabilities	2,910.5		2,976.5
Total equity	1,810.3		1,886.0
Total liabilities and equity	4,720.8		4,862.5
Reportable legal entities | Non-Guarantor Subsidiaries
Condensed consolidating balance sheet
Cash and cash equivalents	34.2		30.9	$ 37.2	$ 14.6
Accounts receivable, net	518.3		431.1
Inventories	1,885.7		2,009.1
Other current assets	69.6		57.7
Total current assets	2,507.8		2,528.8
Property and equipment, net	1,141.0		1,072.4
Intangible assets	835.8		799.9
Equity method investments	64.4		69.7
Other long-term assets	8.7		4.7
Total assets	4,557.7		4,475.5
Floor plan notes payable	975.8		1,070.8
Floor plan notes payable - non-trade	633.9		620.1
Accounts payable	454.6		443.3
Accrued expenses	858.9		820.9
Current portion of long-term debt	87.7		67.3
Total current liabilities	3,010.9		3,022.4
Long-term debt	243.9		239.6
Deferred tax liabilities	2.9		1.4
Other long-term liabilities	483.3		475.6
Total liabilities	3,741.0		3,739.0
Total equity	816.7		736.5
Total liabilities and equity	$ 4,557.7		$ 4,475.5