Sales of Certain Receivables - Sales of Service Receivables (Details) - USD ($)	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2014
Variable Interest Entity [Line Items]
Other current assets	$ 2,305,000,000	$ 1,598,000,000	$ 1,676,000,000
Accounts payable and accrued liabilities	6,746,000,000	7,676,000,000	7,741,000,000
Other current liabilities	1,673,000,000	$ 815,000,000	787,000,000
Factoring Arrangement | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Revolving receivables facility, maximum borrowing capacity				$ 950,000,000
Revolving receivables facility, outstanding borrowings	924,000,000		774,000,000
Other current assets	350,000,000		339,000,000
Accounts payable and accrued liabilities	25,000,000		59,000,000
Other current liabilities	$ 342,000,000		$ 149,000,000