Summary of Significant Accounting Policies - Narrative (Details) $ in Millions		3 Months Ended								12 Months Ended
	Jan. 01, 2019 USD ($) tower_site	Dec. 31, 2019 USD ($) option	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) option unit	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total imputed discount and allowance rate										7.00%	8.10%
Device upgrade period										18 months
Number of upgrade options | option		1								1
Spectrum license issuance period										15 years
Number of reporting units | unit										2
Federal Universal Service Fund and other fees										$ 93	$ 161	$ 258
Average amortization period, deferred contract costs (in months)		24 months								24 months
Advertising expense										$ 1,600	1,700	1,800
Operating leases		6 years								6 years
Total operating expense										$ 39,276	38,001	35,716
Revenues		$ 11,878	$ 11,061	$ 10,979	$ 11,080	$ 11,445	$ 10,839	$ 10,571	$ 10,455	44,998	43,310	40,604
Net income		751	$ 870	$ 939	$ 908	640	$ 795	$ 782	$ 671	3,468	2,888	4,536
Net cash provided by operating activities										6,824	3,899	3,831
Net cash (used in) provided by financing activities										(2,374)	(3,336)	(1,367)
Accumulated deficit	$ 12,301	8,833				$ 12,954				8,833	12,954
Tower Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property subject to failed sale leaseback transaction, number of units | tower_site	6,200
Other revenues
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										1,164	$ 1,309	$ 1,069
CCI | Tower Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property subject to failed sale leaseback transaction, number of units | tower_site	900
PTI | Tower Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property subject to failed sale leaseback transaction, number of units | tower_site	500
CCI and PTI | Tower Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property subject to failed sale leaseback transaction, number of units | tower_site	1,400
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Average remaining lease term, operating lease	9 years
Operating leases	5 years
Accumulated deficit	$ (653)
Accounting Standards Update 2016-02 | Impact Excluding Failed Sale-Leaseback Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total operating expense										(240)
Accounting Standards Update 2016-02 | Tower Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest expense										(34)
Accounting Standards Update 2016-02 | Impact Including Failed Sale-Leaseback Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total operating expense										(380)
Net income										175
Net cash provided by operating activities										(10)
Net cash (used in) provided by financing activities										10
Accounting Standards Update 2016-02 | Other revenues | Tower Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										(44)
Accounting Standards Update 2016-02 | Other revenues | Impact Including Failed Sale-Leaseback Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										(185)
Minimum | Accounting Standards Update 2016-13 | Pro Forma
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		85								85
Deferred tax assets		(22)								(22)
Accumulated deficit		63								63
Maximum | Accounting Standards Update 2016-13 | Pro Forma
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		95								95
Deferred tax assets		(25)								(25)
Accumulated deficit		$ 70								$ 70
EIP Securitization Arrangement | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equipment installment plan, maximum payment term										24 months
EIP Securitization Arrangement | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equipment installment plan, maximum payment term										36 months