Summary of Significant Accounting Policies - Narrative (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($) option	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) unit option	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total imputed discount and allowances rate										8.10%	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										$ 161,000,000	$ 258,000,000	$ 409,000,000
Advertising expense										1,700,000,000	1,800,000,000	1,700,000,000
Net cash provided by (used in) operating activities										3,899,000,000	3,831,000,000	2,779,000,000
Net cash used in investing activities										(579,000,000)	(6,745,000,000)	(2,324,000,000)
Net cash (used in) provided by financing activities										$ (3,336,000,000)	(1,367,000,000)	463,000,000
Average amortization period, deferred contract costs	24 months									24 months
Revenues	$ 11,445,000,000	$ 10,839,000,000	$ 10,571,000,000	$ 10,455,000,000	$ 10,759,000,000	$ 10,019,000,000	$ 10,213,000,000	$ 9,613,000,000		$ 43,310,000,000	40,604,000,000	37,490,000,000
Interest expense										835,000,000	1,111,000,000	1,418,000,000
Depreciation and amortization										6,486,000,000	5,984,000,000	6,243,000,000
Assets	72,468,000,000				70,563,000,000					72,468,000,000	70,563,000,000
Total operating expense										38,001,000,000	35,716,000,000	33,440,000,000
Net income	$ 640,000,000	$ 795,000,000	$ 782,000,000	$ 671,000,000	$ 2,707,000,000	$ 550,000,000	$ 581,000,000	$ 698,000,000		2,888,000,000	4,536,000,000	1,460,000,000
Other revenues
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										1,309,000,000	1,069,000,000	919,000,000
Service
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										31,992,000,000	30,160,000,000	27,844,000,000
Cost of services and equipment sales										6,307,000,000	6,100,000,000	5,731,000,000
Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income										2,888,000,000	4,536,000,000	1,460,000,000
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment for adoption of new accounting standard														$ 213,000,000
Accounting Standards Update 2014-09 | Contract acquisition costs paid on open contracts
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred contract cost asset														150,000,000
Accounting Standards Update 2014-09 | Contracts with promotional bill credits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred contract cost asset														140,000,000
Accounting Standards Update 2016-15
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by (used in) operating activities										(5,400,000,000)	(4,300,000,000)	(3,400,000,000)
Net cash used in investing activities										5,400,000,000	4,300,000,000	3,400,000,000
Accounting Standards Update 2016-15 | Adjustment for Debt Prepayments and Debt Extinguishment Costs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by (used in) operating activities										(212,000,000)	(188,000,000)	$ 0
Net cash (used in) provided by financing activities										$ 212,000,000	$ 188,000,000
Accounting Standards Update 2016-01 | Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment for adoption of new accounting standard														8,000,000
Accounting Standards Update 2016-16 | Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment for adoption of new accounting standard														$ 11,000,000
Accounting Standards Update 2016-02 | Scenario, Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Average remaining lease term, operating lease													9 years
Accounting Standards Update 2016-02 | Scenario, Forecast | Subsequent Event
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Average remaining lease term for cell sites													4 years
Accounting Standards Update 2016-02 | Scenario, Forecast | Minimum | Subsequent Event | Impact Excluding Failed Sale-Leaseback Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets													$ 8,500,000,000
Liabilities													8,200,000,000
Total operating expense									$ 190,000,000
Net income									140,000,000
Accounting Standards Update 2016-02 | Scenario, Forecast | Minimum | Subsequent Event | Impact Including Failed Sale-Leaseback Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by (used in) operating activities									(20,000,000)
Net cash (used in) provided by financing activities									(20,000,000)
Interest expense									(200,000,000)
Assets													9,100,000,000
Liabilities													7,000,000,000
Total operating expense									220,000,000
Net income									140,000,000
Accounting Standards Update 2016-02 | Scenario, Forecast | Maximum | Subsequent Event | Impact Excluding Failed Sale-Leaseback Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets													9,400,000,000
Liabilities													8,900,000,000
Total operating expense									230,000,000
Net income									180,000,000
Accounting Standards Update 2016-02 | Scenario, Forecast | Maximum | Subsequent Event | Impact Including Failed Sale-Leaseback Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by (used in) operating activities									(40,000,000)
Net cash (used in) provided by financing activities									(40,000,000)
Interest expense									(240,000,000)
Assets													10,000,000,000
Liabilities													7,500,000,000
Total operating expense									260,000,000
Net income									180,000,000
Accounting Standards Update 2016-02 | Scenario, Forecast | Other revenues | Minimum | Subsequent Event | Impact Including Failed Sale-Leaseback Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues									(230,000,000)
Accounting Standards Update 2016-02 | Scenario, Forecast | Other revenues | Maximum | Subsequent Event | Impact Including Failed Sale-Leaseback Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues									$ (250,000,000)
Accounting Standards Update 2016-02 | Accumulated Deficit | Scenario, Forecast | Minimum | Subsequent Event | Impact Excluding Failed Sale-Leaseback Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment for adoption of new accounting standard													(300,000,000)
Accounting Standards Update 2016-02 | Accumulated Deficit | Scenario, Forecast | Minimum | Subsequent Event | Impact Including Failed Sale-Leaseback Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment for adoption of new accounting standard													2,100,000,000
Accounting Standards Update 2016-02 | Accumulated Deficit | Scenario, Forecast | Maximum | Subsequent Event | Impact Excluding Failed Sale-Leaseback Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment for adoption of new accounting standard													(500,000,000)
Accounting Standards Update 2016-02 | Accumulated Deficit | Scenario, Forecast | Maximum | Subsequent Event | Impact Including Failed Sale-Leaseback Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment for adoption of new accounting standard													$ 2,500,000,000
EIP Securitization Arrangement
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equipment installment plan, maximum payment term										36 months