Financial Instruments (Details) In Millions, unless otherwise specified								3 Months Ended	12 Months Ended	1 Months Ended			3 Months Ended			3 Months Ended				3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended			3 Months Ended	12 Months Ended
	Mar. 31, 2012 D	Mar. 31, 2012 U.S. Fleet Variable Funding Notes USD ($)	Dec. 31, 2011 U.S. Fleet Variable Funding Notes USD ($)	Mar. 31, 2012 Australian Securitization USD ($)	Dec. 31, 2011 Australian Securitization USD ($)	Mar. 31, 2012 Unleaded gasoline swaps Gallon	Mar. 31, 2012 Diesel swaps Gallon	Mar. 31, 2012 Interest rate caps	Dec. 31, 2011 Interest rate caps U.S. Fleet Variable Funding Notes USD ($)	Dec. 31, 2010 Interest rate caps Australian Securitization USD ($)	Dec. 31, 2010 Interest rate caps Australian Securitization AUD	Mar. 31, 2012 Foreign exchange forward contracts USD ($)	Mar. 31, 2012 Interest rate swaps Donlen Corporation USD ($)	Mar. 31, 2012 Foreign exchange options USD ($)	Dec. 31, 2011 Foreign exchange options USD ($)	Mar. 31, 2012 Level 1 USD ($)	Dec. 31, 2011 Level 1 USD ($)	Mar. 31, 2012 Level 2 USD ($)	Dec. 31, 2011 Level 2 USD ($)	Mar. 31, 2012 Recurring Derivatives not designated as hedging instruments USD ($)	Dec. 31, 2011 Recurring Derivatives not designated as hedging instruments USD ($)	Mar. 31, 2012 Recurring Derivatives not designated as hedging instruments Gasoline swaps USD ($)	Dec. 31, 2011 Recurring Derivatives not designated as hedging instruments Gasoline swaps USD ($)	Mar. 31, 2012 Recurring Derivatives not designated as hedging instruments Interest rate caps USD ($)	Dec. 31, 2011 Recurring Derivatives not designated as hedging instruments Interest rate caps USD ($)	Mar. 31, 2012 Recurring Derivatives not designated as hedging instruments Foreign exchange forward contracts USD ($)	Dec. 31, 2011 Recurring Derivatives not designated as hedging instruments Foreign exchange forward contracts USD ($)	Mar. 31, 2012 Recurring Derivatives not designated as hedging instruments Interest rate swaps USD ($)	Dec. 31, 2011 Recurring Derivatives not designated as hedging instruments Interest rate swaps USD ($)	Mar. 31, 2012 Recurring Derivatives not designated as hedging instruments Foreign exchange options USD ($)	Dec. 31, 2011 Recurring Derivatives not designated as hedging instruments Foreign exchange options USD ($)	Mar. 31, 2012 Recurring Level 2 USD ($)	Dec. 31, 2011 Recurring Level 2 USD ($)
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents
Money market accounts included in cash and cash equivalents at fair value																$ 146.3	$ 566.0
Money market accounts included in restricted cash and cash equivalents at fair value																194.9	142.9
Marketable Securities
Available-for-sale marketable equity securities included in prepaid expenses and other assets at fair value																38.2	33.2
Unrealized gains on marketable equity securities net of income taxes included in accumulated other comprehensive income																5.0
Debt
Maximum initial maturity period of borrowings (in days)	93
Aggregate fair value of all debt																		12,228.9	11,832.5
Aggregate carrying value of all debt		1,720.0	1,345.0	165.2	169.3																											11,491.2	11,400.3
Fair Value of Derivative Instruments
Asset Derivatives																				7.9	5.0	1.2		0.1	0.5	6.5	4.4			0.1	0.1
Liability Derivatives																				3.0	2.9		0.4	0.1	0.4	2.8	1.9	0.1	0.2
Amount of Gain or (Loss) Recognized in Income on Derivative																				(3.6)	2.5	1.8	3.1			(5.4)	(0.6)
Derivative purchased, amount									6.7	0.5
Combined maximum principal amount									2,100		250
Strike rate (as a percent)									5.00%		7.00%
Derivative sold, amount									6.2	0.4
Variable rate used to calculate fair values of all interest rate caps								1-month LIBOR					1-month LIBOR
Total notional amount												578.8	25.6	8.6	9.1
Outstanding commodity instruments (in gallons)						8,200,000	600,000
Premiums paid for options outstanding														$ 0.2
Maturity period (in months)												4 months