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Intangible Assets (Intangible Assets Subject to Amortization) (Details) - USD ($)
$ in Millions
12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Finite-Lived Intangible Assets [Line Items]    
Gross carrying value $ 8,402 $ 8,992
Accumulated amortization (5,937) (5,672)
Net carrying value 2,465 3,320
Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross carrying value 6,562 6,923
Accumulated amortization (5,462) (5,053)
Net carrying value 1,100 1,870
Favorable spectrum leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross carrying value 856 869
Accumulated amortization (172) (138)
Net carrying value 684 731
Favorable tower leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross carrying value 335 589
Accumulated amortization (179) (386)
Net carrying value 156 203
Trademarks [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross carrying value 520 520
Accumulated amortization (74) (58)
Net carrying value 446 462
Other Finite Lived Intangible Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross carrying value 129 91
Accumulated amortization (50) (37)
Net carrying value 79 54
Finite Lived Intangible Assets Excluding Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross carrying value 1,840 2,069
Accumulated amortization (475) (619)
Net carrying value $ 1,365 $ 1,450
Minimum [Member] | Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Useful lives 5 years  
Minimum [Member] | Favorable tower leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Useful lives  
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 7 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