Other Financial Information (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Prepaid Expenses and Other Current Assets [Abstract]
Prepaid expenses	$ 386	$ 307
Income tax receivable	69	82
Materials, supplies and inventory	154	120
Contract assets	58	52
Other	98	108
Total other current assets	1,026	918
Contract Acquisition Costs
Prepaid Expenses and Other Current Assets [Abstract]
Contract costs	173	167
Contract Fulfillment Costs
Prepaid Expenses and Other Current Assets [Abstract]
Contract costs	$ 88	$ 82