Accounts Receivable (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Accounts receivable
Other				$ 3,000,000	$ 4,000,000
Total accounts receivable, gross, current				693,000,000	753,000,000
Accounts receivable, allowance (in dollars)	$ (53,000,000)	$ (47,000,000)	$ (38,000,000)	(47,000,000)	(53,000,000)
Accounts receivable, less allowance				646,000,000	700,000,000
Customer receivable in excess of 10% of accounts receivable				0
Changes in allowance for doubtful accounts
Beginning balance	53,000,000	47,000,000	38,000,000
Provision for uncollectible accounts	74,000,000	80,000,000	78,000,000
Deductions	(80,000,000)	(74,000,000)	(69,000,000)
Ending balance	$ 47,000,000	$ 53,000,000	$ 47,000,000
Earned and unbilled receivables
Accounts receivable
Total accounts receivable, gross, current				99,000,000	115,000,000
Trade and purchased receivables
Accounts receivable
Total accounts receivable, gross, current				$ 591,000,000	$ 634,000,000