Accounts Receivable (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Accounts receivable
Accounts receivable, gross, current				$ 735,000,000	$ 778,000,000
Other				4,000,000	9,000,000
Total accounts receivable, gross, current				735,000,000	778,000,000
Accounts receivable, allowance (in dollars)	$ (38,000,000)	$ (43,000,000)	$ (46,000,000)	(47,000,000)	(38,000,000)
Accounts receivable, less allowance				688,000,000	740,000,000
Customer receivable in excess of 10% of accounts receivable				0
Changes in allowance for doubtful accounts
Beginning balance	38,000,000	43,000,000	46,000,000
Provision for uncollectible accounts	78,000,000	64,000,000	65,000,000
Deductions	(69,000,000)	(69,000,000)	(68,000,000)
Ending balance	$ 47,000,000	$ 38,000,000	$ 43,000,000
Unbilled revenues
Accounts receivable
Accounts receivable, gross, current				111,000,000	120,000,000
Total accounts receivable, gross, current				111,000,000	120,000,000
Trade accounts receivable
Accounts receivable
Accounts receivable, gross, current				620,000,000	649,000,000
Total accounts receivable, gross, current				$ 620,000,000	$ 649,000,000