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Fair Value Measurements - Narrative (Details) - USD ($)	3 Months Ended		12 Months Ended
Fair Value Measurements - Narrative (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Oct. 28, 2020	Oct. 06, 2020	Jun. 24, 2020	Apr. 09, 2020	Apr. 06, 2020	Oct. 31, 2018
Derivative [Line Items]
Net collateral transfers to certain derivative counterparties	$ (580,000,000)	$ (632,000,000)	$ (632,000,000)	$ 632,000,000	$ 0
Accumulated other comprehensive loss		868,000,000	1,581,000,000	868,000,000
Principal Issuances			54,750,000,000
Guarantee liabilities		62,000,000	50,000,000	62,000,000
Total EIP Receivables, net of unamortized imputed discounts		4,582,000,000	6,213,000,000	4,582,000,000
Interest Expense
Derivative [Line Items]
Amount amortized from AOCI into Interest expense			128,000,000	0	$ 0
Amount expected to be amortized from AOCI into interest expense over next 12 months			189,000,000
Equipment installment plan receivables, net of allowance for credit losses and imputed discount
Derivative [Line Items]
Total EIP Receivables, net of unamortized imputed discounts			3,400,000,000
Level 3 \| Fair Value
Derivative [Line Items]
Carrying amounts of deferred purchase price assets		781,000,000	884,000,000	781,000,000
Senior Secured Notes due 2025, 3.500%
Derivative [Line Items]
Principal Issuances			3,000,000,000
Interest rate, stated percentage									3.50%
Senior Secured Notes due 2050, 4.500%
Derivative [Line Items]
Principal Issuances			$ 3,000,000,000
Interest rate, stated percentage									4.50%
Senior Notes
Derivative [Line Items]
Principal Issuances						$ 4,750,000,000	$ 4,000,000,000.0	$ 4,000,000,000.0	$ 19,000,000,000.0
Interest Rate Contract
Derivative [Line Items]
Derivative liabilities										$ 2,300,000,000
Cash-collateralized										$ 1,200,000,000
Interest Rate Contract \| Cash Flow Hedging
Derivative [Line Items]
Aggregate notional amount											$ 9,600,000,000
Fair value of derivative instrument		$ 1,200,000,000		$ 1,200,000,000