Business Combinations - Narrative (Details) - USD ($)					3 Months Ended		6 Months Ended
	May 18, 2018	Apr. 29, 2018	Jan. 22, 2018	Jan. 01, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 06, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Payments of consent fees					$ 38,000,000	$ 0	$ 38,000,000	$ 0
Payments for third party bank fees					3,185,000,000	$ 2,915,000,000	6,349,000,000	$ 5,870,000,000
Goodwill					1,901,000,000		1,901,000,000			$ 1,683,000,000
Sprint
Business Acquisition [Line Items]
Exchange ratio		0.10256
Exchange ratio (in shares)		9.75
Fully-diluted shares of combined company held by public stockholders (percent)		30.90%
Payments of consent fees					31,000,000
Required reimbursement by acquiree upon termination, percentage	33.00%
Upfront payments to third party note holders by acquiree					241,000,000
Required reimbursement by acquirer upon termination, percentage	67.00%
Required reimbursement by acquirer upon termination	$ 161,000,000
Costs recognized associated with merger transaction					41,000,000		41,000,000
Required payment resulting from failure to satisfy closing condition of business combination agreement (up to)		$ 600,000,000
Sprint | Senior Notes
Business Acquisition [Line Items]
Payments of consent fees	$ 14,000,000				17,000,000
Payments for third party bank fees					6,000,000
Sprint | Secured and Unsecured Debt Financing
Business Acquisition [Line Items]
Financing commitment, amount		$ 38,000,000,000			30,000,000,000		$ 30,000,000,000
Reduction of initial aggregate commitment under Commitment Letter									$ (8,000,000,000)
Sprint | Existing Sprint Spectrum Notes | Senior Notes
Business Acquisition [Line Items]
Payments of consent fees					14,000,000
Sprint | Deutsche Telekom AG
Business Acquisition [Line Items]
Fully-diluted shares expected to be held immediately following merger (percent)		41.70%
Sprint | SoftBank
Business Acquisition [Line Items]
Fully-diluted shares expected to be held immediately following merger (percent)		27.40%
Layer3 TV
Business Acquisition [Line Items]
Cash consideration			$ 318,000,000
Payment included in consideration after closing of transaction			5,000,000
Grant-date fair value of cash-based and share-based incentive compensation awards			37,000,000
Liability recognized			21,000,000
Indemnification assets			12,000,000
Maximum amount that would be received under the indemnification asset			12,000,000
Goodwill			218,000,000
Intangible assets			$ 100,000,000
Layer3 TV | Developed Technology
Business Acquisition [Line Items]
Estimated useful life (in years)			5 years
Intangible assets			$ 100,000,000
Iowa Wireless
Business Acquisition [Line Items]
Cash consideration				$ 25,000,000
Purchase price of unconsolidated subsidiary				25,000,000
Fair value of equity interest held				56,000,000
Gain on remeasurement of equity interest				15,000,000
Bargain purchase gain				$ 25,000,000
Unconsolidated subsidiary ownership percentage				54.00%
Deutsche Telekom AG | Sprint
Business Acquisition [Line Items]
Payments of consent fees					$ 7,000,000