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Intangible Assets (Intangible Assets Subject to Amortization) (Details) - USD ($)
$ in Millions
12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Finite-Lived Intangible Assets [Line Items]    
Gross carrying value $ 8,318 $ 8,402
Accumulated amortization (6,549) (5,937)
Net carrying value 1,769 2,465
Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross carrying value 6,563 6,562
Accumulated amortization (6,029) (5,462)
Net carrying value 534 1,100
Favorable spectrum leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross carrying value 763 856
Accumulated amortization (150) (172)
Net carrying value 613 684
Favorable tower leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross carrying value 335 335
Accumulated amortization (215) (179)
Net carrying value 120 156
Trademarks [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross carrying value 520 520
Accumulated amortization (89) (74)
Net carrying value 431 446
Other Finite Lived Intangible Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross carrying value 137 129
Accumulated amortization (66) (50)
Net carrying value 71 79
Finite Lived Intangible Assets Excluding Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross carrying value 1,755 1,840
Accumulated amortization (520) (475)
Net carrying value $ 1,235 $ 1,365
Minimum [Member] | Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Useful lives 5 years  
Minimum [Member] | Trademarks [Member]    
Finite-Lived Intangible Assets [Line Items]    
Useful lives 2 years  
Minimum [Member] | Other Finite Lived Intangible Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Useful lives 5 years  
Maximum [Member] | Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Useful lives 8 years  
Maximum [Member] | Favorable spectrum leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Useful lives 23 years  
Maximum [Member] | Favorable tower leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Useful lives 9 years  
Maximum [Member] | Trademarks [Member]    
Finite-Lived Intangible Assets [Line Items]    
Useful lives 34 years  
Maximum [Member] | Other Finite Lived Intangible Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Useful lives 10 years