Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2014	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Nov. 19, 2015
Property, Plant and Equipment [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property			$ 509	$ 487
Gain (Loss) on Disposition of Property Plant Equipment				65
Capital Expenditures Incurred but Not yet Paid			962	468	$ 1,500
Sale Leaseback Transaction, Net Book Value						$ 1,300	$ 1,300
Depreciation			7,098	5,794	3,797
Wireline [Member]
Property, Plant and Equipment [Line Items]
Impairment loss		$ 233
Estimated fair value		$ 918
Leased devices [Member]
Property, Plant and Equipment [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property			481	256
Gain (Loss) on Disposition of Property Plant Equipment				65
Increase (decrease) in device and accessory inventory			2,900	3,200
Increase in accounts payable			158	159	182
Depreciation			3,100	1,800	$ 206
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			$ 28	$ 166
Leased handset [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Additions	$ 477