Summary of Significant Accounting Policies (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Property, Plant and Equipment [Line Items]
Ownership Interest Percent Threshold to Consolidate	50.00%
Contract carrier agreements, number of contract carriers	8
Depreciation, Nonproduction	$ 1,400,000,000	$ 1,400,000,000	$ 1,400,000,000
Capitalized Computer Software, Amortization	76,000,000	64,000,000	71,000,000
Capitalized Computer Software, Net	344,000,000	200,000,000
Goodwill	9,794,000,000	9,794,000,000
Intangible Assets, Net (Excluding Goodwill)	4,679,000,000	4,751,000,000
Advertising Expense	$ 235,000,000	$ 214,000,000	$ 169,000,000
Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Estimated Useful Lives	3
Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Estimated Useful Lives	7
Maximum [Member]
Property, Plant and Equipment [Line Items]
Residual Values For Owned Spare Parts As Percentage Of Cost	10.00%
Minimum [Member]
Property, Plant and Equipment [Line Items]
Residual Values For Owned Spare Parts As Percentage Of Cost	5.00%