Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Property, Plant and Equipment [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 1,183	$ 704	$ 962
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property	1,135	868	509
Gain (Loss) on Disposition of Property Plant Equipment		6
Accrued capital expenditures	962	0
Leased devices [Member]
Property, Plant and Equipment [Line Items]
Noncash or Part Noncash Acquisition, Fixed Assets Acquired	879	661
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property	643	493	481
Accrued capital expenditures	185	256	158
Network Equipment, Site Costs and Related Software [Member]
Property, Plant and Equipment [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property	$ 492	375	$ 28
Gain (Loss) on Disposition of Property Plant Equipment		$ 370