Sales of Certain Receivables - Sales of EIP Receivables (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2015
Variable Interest Entity [Line Items]
Other current assets	$ 2,882,000,000	$ 2,305,000,000
Other assets	2,026,000,000	1,891,000,000
EIP Securitization Arrangement
Variable Interest Entity [Line Items]
Revolving receivables facility, maximum borrowing capacity	1,300,000,000	1,300,000,000	$ 1,300,000,000
Other current assets	335,000,000	344,000,000
Other assets	92,000,000	89,000,000
Other long-term liabilities	$ 8,000,000	$ 18,000,000