New Accounting Pronouncements - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2008 towers	Sep. 30, 2019 USD ($)	Apr. 01, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of Tower Sites Transferred To Third Party | towers	1,750
Operating Lease, Right-of-Use Asset		$ 6,885	$ 7,358
Operating Lease, Liability, Current		1,835	1,813
Operating Lease, Liability, Noncurrent		5,667	6,263
Finance lease, right-of-use asset		$ 612	613
Accumulated Deficit [Member] | Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption			104
Liability [Member] | Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption			108
Property, Plant and Equipment [Member] | Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption			$ 4