Commitments and Contingencies (Narrative) (Details) $ in Millions		6 Months Ended
	Apr. 22, 2019 defendant	Sep. 30, 2019 USD ($) claim	Mar. 31, 2019 USD ($)
Commitments and Contingencies [Line Items]
Loss Contingency, Number of Defendants | defendant	2
Minimum spectrum reconfiguration obligations | $		$ 2,800
Total Payments directly attributable to performance under Report and Order | $		3,600
Payments directly attributable to performance under report and order, net change related to FCC licenses | $		7
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 | $		$ 74
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