Basis of Presentation Basis of Presentation (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2015
Change in Accounting Estimate [Line Items]
Change in net income for change in accounting estimate	$ (192)	$ (203)
Unamortized debt issuance costs	172
Network assets, future abandonment | Service life
Change in Accounting Estimate [Line Items]
Depreciation	12
Change in net income for change in accounting estimate	(7)
Earnings per share, basic and diluted	$ (0.01)
Network assets, future abandonment | Forecast | Service life
Change in Accounting Estimate [Line Items]
Depreciation			48
Change in net income for change in accounting estimate			$ (30)
Earnings per share, basic and diluted			$ (0.05)