Equity Method Investees (Details) - USD ($) $ in Millions				1 Months Ended							3 Months Ended	12 Months Ended
	Sep. 07, 2017	Jul. 27, 2016	Jul. 26, 2016	Dec. 31, 2017	Sep. 30, 2017	May 31, 2017	Sep. 30, 2016	Jul. 31, 2016	Jun. 30, 2016	Sep. 30, 2015	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity Method Investment
Ownership acquired (as a percent)					5.50%			14.40%
Equity method investments				$ 1,256.6								$ 1,256.6	$ 893.4
Ownership interest in Penske Truck Leasing Co (as a percent)								23.40%	9.00%			28.90%
Equity in earnings of affiliates												$ 107.6	69.5	$ 39.3
Condensed income statement information
Revenues												7,680.8	7,397.2	6,770.3
Gross profit												1,792.4	1,670.4	2,396.8
Net income												416.1	411.5	$ 398.5
Condensed balance sheet information
Current assets				1,317.9								1,317.9	1,188.2
Noncurrent assets				11,729.4								11,729.4	10,529.9
Total assets				13,047.3								13,047.3	11,718.1
Current liabilities				2,101.0								2,101.0	1,794.7
Noncurrent liabilities				8,160.3								8,160.3	7,393.6
Equity				2,786.0								2,786.0	2,529.8
Total liabilities and equity				$ 13,047.3								$ 13,047.3	11,718.1
The Nix Group
Equity Method Investment
Ownership percentage of equity method investment				50.00%								50.00%
Ibericar Keldinich SL
Equity Method Investment
Ownership percentage of equity method investment				50.00%								50.00%
Nicole Group
Equity Method Investment
Ownership percentage of equity method investment				49.00%								49.00%
Penske Commercial Leasing Australia
Equity Method Investment
Ownership percentage of equity method investment				28.00%								28.00%
Ownership interest sold (as a percent)					5.00%		12.00%
Penske Vehicle Services
Equity Method Investment
Ownership interest sold (as a percent)				31.00%
National Powersport Auctions
Equity Method Investment
Ownership interest sold (as a percent)						7.00%
Max Cycles
Equity Method Investment
Ownership interest sold (as a percent)										50.00%
PTL
Equity Method Investment
Ownership percentage of equity method investment				28.90%								28.90%
Ownership acquired (as a percent)	5.50%	14.40%			5.50%
Equity method investments				$ 1,185.6								$ 1,185.6	823.8
Ownership interest in Penske Truck Leasing Co (as a percent)		23.40%	9.00%									28.90%
Nix Group, Ibericar Keldinich SL, the Nicole Group, and Penske Commercial Leasing Australia
Equity Method Investment
Equity method investments				$ 71.0								$ 71.0
Nix Group, Ibericar Keldinich SL, the Nicole Group, Penske Commercial Leasing Australia, Penske Vehicle Services and National Powersport Auctions
Equity Method Investment
Equity method investments													$ 69.6
German Automotive Dealership Group
Equity Method Investment
Ownership acquired (as a percent)										10.00%	8.00%
Ownership interest (as a percent)				68.00%								68.00%		60.00%