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Intangible Assets (Intangible Assets Subject to Amortization) (Details) - USD ($)
$ in Millions
6 Months Ended
Sep. 30, 2018
Mar. 31, 2018
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value $ 8,318 $ 8,402
Accumulated Amortization (6,243) (5,937)
Net Carrying Value 2,075 2,465
Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 6,563 6,562
Accumulated Amortization (5,771) (5,462)
Net Carrying Value 792 1,100
Favorable spectrum leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 767 856
Accumulated Amortization (135) (172)
Net Carrying Value 632 684
Favorable tower leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 335 335
Accumulated Amortization (197) (179)
Net Carrying Value 138 156
Trademarks [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 520 520
Accumulated Amortization (82) (74)
Net Carrying Value 438 446
Other Finite Lived Intangible Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 133 129
Accumulated Amortization (58) (50)
Net Carrying Value 75 79
Finite Lived Intangible Assets Excluding Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 1,755 1,840
Accumulated Amortization (472) (475)
Net Carrying Value $ 1,283 $ 1,365
Minimum [Member] | Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 5 years  
Minimum [Member] | Favorable tower leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life  
Minimum [Member] | Trademarks [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 2 years  
Minimum [Member] | Other Finite Lived Intangible Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 5 years  
Maximum [Member] | Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 8 years  
Maximum [Member] | Favorable spectrum leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 23 years  
Maximum [Member] | Favorable tower leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 7 years  
Maximum [Member] | Trademarks [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 34 years  
Maximum [Member] | Other Finite Lived Intangible Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 10 years