Funding Sources Handset Sale-Leaseback (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended	6 Months Ended
	Apr. 30, 2016	Nov. 30, 2015	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	May 31, 2016	Mar. 31, 2016	Dec. 31, 2015	Nov. 19, 2015
Sale Leaseback Transaction [Line Items]
Document Period End Date				Sep. 30, 2016
Sale Leaseback Transaction, Net Book Value			$ 502	$ 502			$ 1,200	$ 1,300
Gain (Loss) on Disposition of Property Plant Equipment					$ 85
Leased handset [Member]
Sale Leaseback Transaction [Line Items]
Sale Leaseback Transaction, Term of Lease				17 months
Sale Leaseback Transaction, Net Proceeds, Investing Activities		$ 1,100
Receivables from sale-leaseback									$ 126
Gain (Loss) on Disposition of Property Plant Equipment		$ 65
Sale Leaseback Transaction, Rent Expense			180	$ 377
Sale Leaseback Transaction, Monthly Rental Payments			60
MLS 2nd Tranche [Member]
Sale Leaseback Transaction [Line Items]
Sale Leaseback Transaction, Net Book Value			758	758		$ 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			$ 900	$ 900