Property, Plant and Equipment (Components of Property Plant and Equipment) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Apr. 01, 2019	Mar. 31, 2019
Property, Plant and Equipment [Line Items]
Document Period End Date			Dec. 31, 2019
Gain (loss) on disposition of property, plant, and equipment, excluding oil and gas property and timber property			$ (692)	$ (642)
Property, Plant and Equipment					$ 41,709
Less accumulated depreciation	$ (19,992)		(19,992)		(20,512)	$ (20,539)
Property, plant and equipment, net	20,827		20,827		$ 21,197	21,201
Land [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	105		105			246
Network equipment, site costs and related software [Member]
Property, Plant and Equipment [Line Items]
Gain (loss) on disposition of property, plant, and equipment, excluding oil and gas property and timber property		$ 117	26	185
Property, Plant and Equipment	25,373		25,373			24,967
Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	444		444			856
Non-network internal use software, office equipment, leased devices and other [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	12,269		12,269			12,627
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Construction in progress	2,628		2,628			$ 3,044
Write-off leased devices [Member]
Property, Plant and Equipment [Line Items]
Gain (loss) on disposition of property, plant, and equipment, excluding oil and gas property and timber property	$ 201	$ 182	$ 666	$ 457