Receivables and Allowance for Credit Losses - Gross EIP Receivables by Credit Category (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	$ 4,582	$ 4,534
Prime
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	2,411	2,015
Subprime
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	2,171	2,519
Current - 30 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	4,492	4,433
Current - 30 days past due | Prime
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	2,384	1,987
Current - 30 days past due | Subprime
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	2,108	2,446
31 - 60 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	41	47
31 - 60 days past due | Prime
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	13	15
31 - 60 days past due | Subprime
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	28	32
61 - 90 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	24	25
61 - 90 days past due | Prime
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	7	6
61 - 90 days past due | Subprime
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	17	19
More than 90 days past due
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	25	29
More than 90 days past due | Prime
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	7	7
More than 90 days past due | Subprime
Financing Receivable, Credit Quality Indicator [Line Items]
EIP receivables, gross	$ 18	$ 22