Organization and Summary of Significant Accounting Policies - Business Overview (Details) $ in Millions			1 Months Ended				3 Months Ended								12 Months Ended
	Sep. 07, 2017	Sep. 06, 2017	Apr. 30, 2018 item	Sep. 30, 2017	Jul. 31, 2016	Jun. 30, 2016	Dec. 31, 2018 USD ($) item	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)
Significant Accounting Policies
Total 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
Number of reportable segments															4
Receivables from manufacturers | $							$ 211.3				$ 230.1				$ 211.3	$ 230.1
Minimum
Significant Accounting Policies
Ownership percentage of equity method investment							20.00%								20.00%
Maximum
Significant Accounting Policies
Ownership percentage of equity method investment							50.00%								50.00%
German Automotive Dealership Group
Significant Accounting Policies
Ownership acquired (as a percent)															11.40%
Ownership interest (as a percent)							79.40%								79.40%
PTL
Significant Accounting Policies
Ownership interest in Penske Truck Leasing Co (as a percent)		23.40%			23.40%	9.00%									28.90%
Ownership acquired (as a percent)	5.50%			5.50%	14.40%
CarSense
Significant Accounting Policies
Number of retail locations acquired																5
CarShop
Significant Accounting Policies
Number of retail locations acquired																5
The Car People
Significant Accounting Policies
Number of retail locations acquired															4
CarShop and the The Car People
Significant Accounting Policies
Number of reportable segments															1
Penske Corporation | PTL
Significant Accounting Policies
Ownership interest in Penske Truck Leasing Co (as a percent)															41.10%
Mitsui and Co | PTL
Significant Accounting Policies
Ownership interest in Penske Truck Leasing Co (as a percent)															30.00%
Ownership acquired (as a percent)	10.00%
Retail Automotive Dealership
Significant Accounting Policies
Total number of owned and operated franchises							345								345
Number of owned and operated franchises in US							154								154
Number of owned and operated franchises outside US							191								191
Number of acquired franchises															6
Number of franchises awarded to the reporting entity															3
Number of franchises disposed															7
Number of stand-alone used vehicle dealerships operated in the U.S. and U.K.							14								14
Retail Automotive Dealership | CarSense
Significant Accounting Policies
Number of retail locations operated															5
Retail Automotive Dealership | CarShop
Significant Accounting Policies
Number of retail locations operated															5
Retail Automotive Dealership | The Car People
Significant Accounting Policies
Number of retail locations operated															4
Retail Commercial Truck Dealership | PTG
Significant Accounting Policies
Number of operating franchises															20
Number of operating locations															20
Number of locations acquired			1
Commercial Vehicle Distribution
Significant Accounting Policies
Minimum number of dealership locations															70
Number of retail dealerships owned by the reporting entity															9
BMW, MINI | Automotive dealership revenues
Significant Accounting Policies
Percentage of total															23.00%
Audi/Volkswagen/Porsche/Bentley | Automotive dealership revenues
Significant Accounting Policies
Percentage of total															24.00%
Toyota/Lexus | Automotive dealership revenues
Significant Accounting Policies
Percentage of total															13.00%
Retail Automotive Dealership
Significant Accounting Policies
Total revenues | $															$ 20,849.2	$ 19,824.3	18,673.2
Number of reportable segments															1
Retail Commercial Truck Dealership
Significant Accounting Policies
Total revenues | $															$ 1,374.5	1,048.0	1,000.7
Commercial Vehicle Distribution and Other
Significant Accounting Policies
Total revenues | $															$ 561.4	514.6	444.6
Italy | Retail Automotive Dealership
Significant Accounting Policies
Number of acquired franchises															4
Texas | Retail Automotive Dealership
Significant Accounting Policies
Number of acquired franchises															2
UK | Retail Automotive Dealership
Significant Accounting Policies
Number of franchises disposed															2
UK | Retail Automotive Dealership
Significant Accounting Policies
Total revenues | $															$ 7,961.4
U.S.
Significant Accounting Policies
Total revenues | $															$ 12,607.8	$ 12,487.2	$ 12,005.9
U.S. | Retail Automotive Dealership
Significant Accounting Policies
Number of franchises disposed															5
U.S. | Retail Automotive Dealership
Significant Accounting Policies
Total revenues | $															$ 11,504.3