Accounts Receivable (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Accounts receivable
Other				$ 4,000,000	$ 3,000,000
Total accounts receivable, gross, current				587,000,000	693,000,000
Less: allowance for doubtful accounts	$ (47,000,000)	$ (53,000,000)	$ (47,000,000)	(41,000,000)	(47,000,000)
Accounts receivable, less allowance				546,000,000	646,000,000
Customer receivable in excess of 10% of accounts receivable				0
Changes in allowance for doubtful accounts
Beginning balance	47,000,000	53,000,000	47,000,000
Additions	60,000,000	74,000,000	80,000,000
Deductions	(66,000,000)	(80,000,000)	(74,000,000)
Ending balance	$ 41,000,000	$ 47,000,000	$ 53,000,000
Earned and unbilled receivables
Accounts receivable
Total accounts receivable, gross, current				92,000,000	99,000,000
Trade and purchased receivables
Accounts receivable
Total accounts receivable, gross, current				$ 491,000,000	$ 591,000,000