Commitments and Contingencies (Narrative) (Details) $ in Millions		3 Months Ended	9 Months Ended
	Apr. 19, 2012 USD ($)	Dec. 31, 2018 USD ($) claim	Dec. 31, 2018 USD ($) claim	Mar. 31, 2018 USD ($)
Commitments and Contingencies [Line Items]
Minimum spectrum reconfiguration obligations		$ 2,800	$ 2,800
Total Payments directly attributable to performance under Report and Order		3,600	3,600
Payments directly attributable to performance under report and order, net change related to FCC licenses			35
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		$ 78	$ 78
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
Litigation Settlement, Expense		$ 50
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 Accrual, Provision			$ 114