Interim Financial Statements - Business Overview (Details) $ in Millions	1 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
	Jul. 31, 2019 location	Sep. 30, 2019 USD ($) item	Mar. 31, 2019	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Interim Financial Statements
Total revenues | $		$ 5,967.6		$ 5,658.6	$ 17,287.8	$ 17,345.8
Minimum number of vehicle brands offered					35
Number of used vehicle supercenters operated in the U.S. and U.K.		15			15
PTL
Interim Financial Statements
Ownership interest in Penske Truck Leasing Co (as a percent)					28.90%
German Automotive Dealership Group
Interim Financial Statements
Ownership acquired (as a percent)			8.40%
Ownership interest (as a percent)			87.80%
Retail Automotive Dealership
Interim Financial Statements
Total number of owned and operated franchises		333			333
Number of owned and operated franchises in US		149			149
Number of owned and operated franchises outside US		184			184
Minimum number of vehicles retailed and wholesaled					478,000
Number of franchises awarded to the reporting entity					1
Number of franchises disposed					13
Commercial Vehicle Distribution
Interim Financial Statements
Minimum number of dealership locations					70
Number of retail dealerships owned by the reporting entity					10
UK | Retail Automotive Dealership
Interim Financial Statements
Number of franchises disposed					1
U.S. | Retail Automotive Dealership
Interim Financial Statements
Number of franchises disposed					6
Germany | Retail Automotive Dealership
Interim Financial Statements
Number of franchises disposed					6
Automotive dealership revenues | Geographic | U.S. and Puerto Rico
Interim Financial Statements
Automotive dealership revenue (as a percent)					56.00%
Automotive dealership revenues | Geographic | Outside the U.S.
Interim Financial Statements
Automotive dealership revenue (as a percent)					44.00%
Automotive dealership revenues | Premium brands
Interim Financial Statements
Automotive dealership revenue (as a percent)					70.00%
CarSense | Retail Automotive Dealership
Interim Financial Statements
Number of retail locations operated					6
CarShop | Retail Automotive Dealership
Interim Financial Statements
Number of retail locations operated					9
Penske Corporation | PTL
Interim Financial Statements
Ownership interest in Penske Truck Leasing Co (as a percent)					41.10%
Mitsui and Co | PTL
Interim Financial Statements
Ownership interest in Penske Truck Leasing Co (as a percent)					30.00%
PTG
Interim Financial Statements
Number of locations acquired | location	6
PTG | Retail Commercial Truck Dealership
Interim Financial Statements
Number of operating locations					25
Retail Automotive Dealership
Interim Financial Statements
Total revenues | $		$ 5,155.4		5,148.5	$ 15,442.9	15,900.0	$ 20,800.0
Retail Automotive Dealership | UK
Interim Financial Statements
Total revenues | $		1,859.6		1,945.1	5,827.7	6,244.9
Retail Automotive Dealership | U.S.
Interim Financial Statements
Total revenues | $		2,995.5		2,891.6	8,624.1	8,596.4
Retail Commercial Truck Dealership
Interim Financial Statements
Total revenues | $		692.3		385.3	1,451.4	1,016.5
Commercial Vehicle Distribution and Other
Interim Financial Statements
Total revenues | $		$ 119.9		$ 124.8	$ 393.5	$ 429.3