Depreciation And Amortization (Details) (USD $)	Mar. 31, 2012	Dec. 31, 2011
Depreciation And Amortization [Abstract]
Accumulated depreciation of owned equipment and property	$ 10,300,000,000	$ 10,100,000,000
Accumulated amortization of equipment and property under capital leases	$ 199,000,000	$ 448,000,000