Funding Sources Handset Sale-Leaseback (Details) - USD ($) $ in Millions	1 Months Ended			3 Months Ended	9 Months Ended
	Dec. 31, 2016	Apr. 30, 2016	Nov. 30, 2015	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	May 31, 2016	Mar. 31, 2016	Nov. 19, 2015
Sale Leaseback Transaction [Line Items]
Sale Leaseback Transaction, Net Book Value	$ 447			$ 447	$ 447	$ 1,300		$ 1,200
Sale Leaseback Transaction, Net Proceeds, Investing Activities					$ 0	1,136
Document Period End Date					Dec. 31, 2016
Loss on disposition of property, plant and equipment				28		(85)
Leased handset [Member]
Sale Leaseback Transaction [Line Items]
Sale Leaseback Transaction, Net Proceeds, Investing Activities			$ 1,100
Receivables from sale-leaseback									$ 126
Property, Plant and Equipment, Additions	477
Additions to Other Assets, Amount	16
Sale Leaseback Transaction, Rent Expense				117	$ 494
Sale Leaseback Transaction, Cash Payment For Retirement	371
Sale Leaseback Transaction, Monthly Rental Payments				60
MLS 2nd Tranche [Member]
Sale Leaseback Transaction [Line Items]
Sale Leaseback Transaction, Net Book Value	554			554	554		$ 1,300
Sale Leaseback Transaction, Net Proceeds, Investing Activities		$ 1,100
Receivables from sale-leaseback		$ 186
Handset Sale-Leaseback Tranche 2 [Member]
Sale Leaseback Transaction [Line Items]
Sale Leaseback Transaction, Net Book Value	$ 725			$ 725	$ 725
Loss on sale of devices to MLS under Tranche 1 [Member]
Sale Leaseback Transaction [Line Items]
Loss on disposition of property, plant and equipment						$ 65