Commitments and Contingencies (Narrative) (Details) $ in Millions		12 Months Ended
	Apr. 19, 2012 USD ($)	Mar. 31, 2018 USD ($) renewal_option claim	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)
Commitments and Contingencies [Line Items]
Minimum spectrum reconfiguration obligations		$ 2,800
Total Payments directly attributable to performance under Report and Order		3,600
Payments directly attributable to performance under report and order, net change		$ 15
Noncancelable portion of leases, maximum		15 years
Number of operating lease renewals available | renewal_option		1
Operating lease, maximum contract length to become eligible for renewal contract, term		10 years
General initial term of operating lease, minimum		5 years
General initial term of operating lease, maximum		12 years
Number of cell site renewal options available | renewal_option		5
Years for each renewal option for cell site leases		5 years
Operating leases, rent expense, net		$ 2,700	$ 3,100	$ 2,900
Maximum length of spectrum lease agreements		30 years
Payment to satisfy other spectrum commitments		$ 6
Maximum remaining commitments of other spectrum commitments		$ 77
Minimum [Member]
Commitments and Contingencies [Line Items]
Remaining other spectrum commitments expected to be incurred over term		15 years
Maximum [Member]
Commitments and Contingencies [Line Items]
Remaining other spectrum commitments expected to be incurred over term		30 years
Letter Of Credit Required By The FCCs Report And Order [Member] | Unsecured Revolving Credit Facility expires in 2018 [Member]
Commitments and Contingencies [Line Items]
Initial letter of credit requirement		$ 2,500
Letters of credit outstanding, amount		$ 115
Stockholder Derivative Suit 2009 [Member]
Commitments and Contingencies [Line Items]
Loss Contingency, Pending Claims, Number | claim		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
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
State and Local Jurisdiction [Member]
Commitments and Contingencies [Line Items]
Loss Contingency, Loss in Period		$ 114
Settled Litigation [Member]
Commitments and Contingencies [Line Items]
Litigation Settlement, Amount Awarded from Other Party		$ 350