Guarantor and Non-Guarantor Condensed Consolidating Financial Statements (Narrative) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Capital asset expenditures, non-vehicle	$ 72	$ 121
Cash flows from investing activities	(1,929)	(2,868)
Cash flows from operating activities	1,014	1,161
Prepaid Expenses and Other Assets | Prior Period Misstatements Corrected in Current Period
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Prior period reclassification adjustment	453
Guarantor Subsidiaries
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Capital asset expenditures, non-vehicle	8	0
Cash flows from investing activities	(44)	28
Cash flows from operating activities	40	(13)
Non-Guarantor Subsidiaries
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Capital asset expenditures, non-vehicle	23	34
Cash flows from investing activities	(2,572)	(3,084)
Cash flows from operating activities	3,316	2,663
Previously reported | Guarantor Subsidiaries | Misclassification of Capital Assets Expenditures, Non-Vehicle
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Cash flows from operating activities		$ (15)
Restatement Adjustment | Non-Guarantor Subsidiaries | Misclassification of Capital Assets Expenditures, Non-Vehicle
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Cash flows from operating activities	$ 15