Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Nov. 19, 2015
Property, Plant and Equipment [Line Items]
Capital Expenditures Incurred but Not yet Paid		$ 704	$ 962	$ 468
Sale Leaseback Transaction, Net Book Value					$ 1,300
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property		868	509	487
Gain (Loss) on Disposition of Property Plant Equipment			65	(166)
Leased devices [Member]
Property, Plant and Equipment [Line Items]
Noncash or Part Noncash Acquisition, Fixed Assets Acquired		661	456
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property		493	481	256
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		375	$ 28	$ 166
Gain (Loss) on Disposition of Property Plant Equipment		$ 370
Leased handset [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Additions	$ 477