Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
	Apr. 19, 2012	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Mar. 31, 2015
Commitments and Contingencies [Line Items]
Loss Contingency Accrual, Provision				$ 91		$ 91
Minimum spectrum reconfiguration obligations		$ 2,800			$ 2,800
Licensees reimbursed share of costs				$ 95
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		26			62
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		256			256
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		$ 21	$ 178		$ 180