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Intangible Assets (Intangible Assets Subject to Amortization) (Details) - USD ($)
$ in Millions
6 Months Ended
Sep. 30, 2019
Apr. 01, 2019
Mar. 31, 2019
Finite-Lived Intangible Assets [Line Items]      
Goodwill, Impairment Loss $ 2,000    
Gross Carrying Value 8,027   $ 8,318
Accumulated Amortization (6,614)   (6,549)
Net Carrying Value 1,413 $ 1,650 1,769
Customer Relationships [Member]      
Finite-Lived Intangible Assets [Line Items]      
Gross Carrying Value 6,563   6,563
Accumulated Amortization (6,236)   (6,029)
Net Carrying Value 327   534
Favorable spectrum leases [Member]      
Finite-Lived Intangible Assets [Line Items]      
Gross Carrying Value 802   763
Accumulated Amortization (207)   (150)
Net Carrying Value 595   613
Favorable tower leases [Member]      
Finite-Lived Intangible Assets [Line Items]      
Gross Carrying Value 0   335
Accumulated Amortization 0   (215)
Net Carrying Value 0   120
Trademarks [Member]      
Finite-Lived Intangible Assets [Line Items]      
Gross Carrying Value 520   520
Accumulated Amortization (97)   (89)
Net Carrying Value 423   431
Other Finite Lived Intangible Assets [Member]      
Finite-Lived Intangible Assets [Line Items]      
Gross Carrying Value 142   137
Accumulated Amortization (74)   (66)
Net Carrying Value 68   71
Finite Lived Intangible Assets Excluding Customer Relationships [Member]      
Finite-Lived Intangible Assets [Line Items]      
Gross Carrying Value 1,464   1,755
Accumulated Amortization (378)   (520)
Net Carrying Value $ 1,086   $ 1,235
Minimum | Customer Relationships [Member]      
Finite-Lived Intangible Assets [Line Items]      
Finite-Lived Intangible Asset, Useful Life 5 years    
Minimum | Favorable tower leases [Member]      
Finite-Lived Intangible Assets [Line Items]      
Finite-Lived Intangible Asset, Useful Life    
Minimum | Trademarks [Member]      
Finite-Lived Intangible Assets [Line Items]      
Finite-Lived Intangible Asset, Useful Life 2 years    
Minimum | Other Finite Lived Intangible Assets [Member]      
Finite-Lived Intangible Assets [Line Items]      
Finite-Lived Intangible Asset, Useful Life 5 years    
Maximum | Customer Relationships [Member]      
Finite-Lived Intangible Assets [Line Items]      
Finite-Lived Intangible Asset, Useful Life 8 years    
Maximum | Favorable spectrum leases [Member]      
Finite-Lived Intangible Assets [Line Items]      
Finite-Lived Intangible Asset, Useful Life 23 years    
Maximum | Favorable tower leases [Member]      
Finite-Lived Intangible Assets [Line Items]      
Finite-Lived Intangible Asset, Useful Life 0 years    
Maximum | Trademarks [Member]      
Finite-Lived Intangible Assets [Line Items]      
Finite-Lived Intangible Asset, Useful Life 34 years    
Maximum | Other Finite Lived Intangible Assets [Member]      
Finite-Lived Intangible Assets [Line Items]      
Finite-Lived Intangible Asset, Useful Life 10 years