Acquisitions (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Customer Relationships	$ 78
Zipcar [Member]
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Date of acquisition	Mar. 14, 2013
Acquisition purchase price	473
Business Acquisition, Purchase Price Allocation, Intangible Assets Other than Goodwill	112
Business Acquisition, Purchase Price Allocation, Goodwill Amount	274
Other intangibles, useful life, in years	8 years
Apex [Member]
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Date of acquisition	Oct. 01, 2012
Business Acquisition, Purchase Price Allocation, Intangible Assets Other than Goodwill	21
Business Acquisition, Purchase Price Allocation, Goodwill Amount	16
Number Of Vehicles	4,000
Business Acquisition, Cost of Acquired Entity, Cash Paid	63
Business Acquisition, Contingent Consideration, Potential Cash Payment	9
Business Acquisition, Contingent Consideration, Potential Cash Payment	26
Business Acquisition, Purchase Price Allocation, Property, Plant and Equipment	$ 33