Depreciation And Amortization (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Depreciation, Depletion and Amortization [Abstract]
Accumulated depreciation of owned equipment and property	$ (10,800,000,000)	$ (10,600,000,000)
Accumulated amortization of equipment and property under capital leases	$ (219,000,000)	$ (205,000,000)