Intangible Assets (Intangible Assets Subject to Amortization) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2019	Apr. 01, 2019	Mar. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Goodwill, Impairment Loss	$ 2,000
Gross Carrying Value	8,029		$ 8,318
Accumulated Amortization	(6,504)		(6,549)
Net Carrying Value	1,525	$ 1,650	1,769
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	6,563		6,563
Accumulated Amortization	(6,139)		(6,029)
Net Carrying Value	424		534
Favorable spectrum leases [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	807		763
Accumulated Amortization	(202)		(150)
Net Carrying Value	605		613
Favorable tower leases [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	0		335
Accumulated Amortization	0		(215)
Net Carrying Value	0		120
Trademarks [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	520		520
Accumulated Amortization	(93)		(89)
Net Carrying Value	427		431
Other Finite Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	139		137
Accumulated Amortization	(70)		(66)
Net Carrying Value	69		71
Finite Lived Intangible Assets Excluding Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	1,466		1,755
Accumulated Amortization	(365)		(520)
Net Carrying Value	$ 1,101		$ 1,235
Minimum | Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	5 years
Minimum | Favorable tower leases [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life
Minimum | Trademarks [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	2 years
Minimum | Other Finite Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	5 years
Maximum | Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	8 years
Maximum | Favorable spectrum leases [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	23 years
Maximum | Favorable tower leases [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	0 years
Maximum | Trademarks [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	34 years
Maximum | Other Finite Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	10 years