Accounts Receivable (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other receivables	$ 40	$ 14
Total accounts receivable	2,721	2,195
Less: allowance for doubtful accounts	(164)	(178)
Accounts receivable, less allowance	2,557	2,017
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	178	152	$ 162
Additions	176	192	177
Deductions	(190)	(166)	(187)
Ending balance	164	178	$ 152
Earned and unbilled receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	436	299
Trade and purchased receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	$ 2,245	$ 1,882