Receivables and Allowance for Credit Losses - Unamortized Imputed Discount and Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses and imputed discount, beginning of period	$ 516	$ 516	$ 482	$ 418
Bad debt expense		307	297	388
Write-offs, net of recoveries		(332)	(328)	(393)
Change in imputed discount on short-term and long-term EIP receivables		136	250	252
Impact on the imputed discount from sales of EIP receivables		(167)	(185)	(183)
Allowance for credit losses and imputed discount, end of period		460	516	482
Accounts Receivable Allowance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses and imputed discount, beginning of period	$ 67	67	86	102
Bad debt expense		77	69	104
Write-offs, net of recoveries		(83)	(88)	(120)
Allowance for credit losses and imputed discount, end of period		$ 61	67	86
EIP Receivables Allowance
Financing Receivable, Allowance for Credit Loss [Line Items]
Weighted average effective imputed interest rate (percentage)	10.00%	8.80%
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses and imputed discount, beginning of period	$ 449	$ 449	396	316
Bad debt expense		230	228	284
Write-offs, net of recoveries		(249)	(240)	(273)
Change in imputed discount on short-term and long-term EIP receivables		136	250	252
Impact on the imputed discount from sales of EIP receivables		(167)	(185)	(183)
Allowance for credit losses and imputed discount, end of period		$ 399	$ 449	$ 396