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Commitments and Contingencies (Narrative) (Details)
$ in Millions
1 Months Ended 3 Months Ended 6 Months Ended
Apr. 19, 2012
USD ($)
Nov. 02, 2017
USD ($)
Sep. 30, 2017
USD ($)
claim
Sep. 30, 2017
USD ($)
claim
Mar. 31, 2017
USD ($)
Commitments and Contingencies [Line Items]          
Minimum spectrum reconfiguration obligations     $ 2,800 $ 2,800  
Document Period End Date       Sep. 30, 2017  
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     1 8  
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     $ 142 $ 142  
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       $ 200  
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  
Subsequent Event [Member] | Settled Litigation [Member]          
Commitments and Contingencies [Line Items]          
Net payment received in settlement   $ 350