Accounts Receivable (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
Accounts receivable
Other				$ 4,000,000	$ 4,000,000
Total accounts receivable, gross, current				753,000,000	735,000,000
Accounts receivable, allowance (in dollars)	$ (47,000,000)	$ (38,000,000)	$ (43,000,000)	(53,000,000)	(47,000,000)
Accounts receivable, less allowance				700,000,000	688,000,000
Customer receivable in excess of 10% of accounts receivable				0
Changes in allowance for doubtful accounts
Beginning balance	47,000,000	38,000,000	43,000,000
Provision for uncollectible accounts	80,000,000	78,000,000	64,000,000
Deductions	(74,000,000)	(69,000,000)	(69,000,000)
Ending balance	$ 53,000,000	$ 47,000,000	$ 38,000,000
Earned and unbilled receivables
Accounts receivable
Total accounts receivable, gross, current				115,000,000	111,000,000
Trade and purchased receivables
Accounts receivable
Total accounts receivable, gross, current				$ 634,000,000	$ 620,000,000