Commitments and Contingencies (Narrative) (Details) $ in Millions		3 Months Ended		9 Months Ended
	Apr. 19, 2012 USD ($)	Dec. 31, 2016 USD ($) claim	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) claim	Mar. 31, 2016 USD ($)
Commitments and Contingencies [Line Items]
Loss Contingency Accrual, Provision			$ 91
Minimum spectrum reconfiguration obligations		$ 2,800		$ 2,800
Document Period End Date				Dec. 31, 2016
Total Payments directly attributable to performance under Report and Order		3,500		$ 3,500
Payments directly attributable to performance under report and order, net change related to FCC licenses		15		43
Unsecured Revolving Credit Facility expires in 2018 [Member] | Letter Of Credit Required By The FCCs Report And Order [Member]
Commitments and Contingencies [Line Items]
Initial Letter of Credit Requirement					$ 2,500
Letters of Credit Outstanding, Amount		$ 189		$ 189
Stockholder Derivative Suit 2009 [Member]
Commitments and Contingencies [Line Items]
Loss Contingency, Pending Claims, Number | claim		5		5
New York sales taxes on receipts from sale of wireless services [Member]
Commitments and Contingencies [Line Items]
Sales receipts alleged to not be reported, amount	$ 100
Loss Contingency Accrual, Provision				$ 180
Derivative Shareholder Suits [Member]
Commitments and Contingencies [Line Items]
Loss Contingency, New Claims Filed, Number | claim				8
Louisiana Municipal Police Employees Retirement System [Member]
Commitments and Contingencies [Line Items]
Loss Contingency, Claims Dismissed, Number | claim				1
Johnson County Kansas [Member]
Commitments and Contingencies [Line Items]
Loss Contingency, New Claims Filed, Number | claim				2
Loss Contingency, Claims Dismissed, Number | claim				1
Federal Court of Kansas [Member]
Commitments and Contingencies [Line Items]
Loss Contingency, Pending Claims, Number | claim		5		5
State and Local Jurisdiction [Member]
Commitments and Contingencies [Line Items]
Loss Contingency, Loss in Period				$ 103