Accounts Receivable (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounts receivable
Trade and purchased receivables	$ 649,000,000	$ 697,000,000
Earned and unbilled receivables	120,000,000	73,000,000
Other	9,000,000	11,000,000
Total accounts receivable	778,000,000	781,000,000
Accounts receivable, allowance (in dollars)	(38,000,000)	(43,000,000)
Accounts receivable, less allowance	740,000,000	738,000,000
Customer receivable in excess of 10% of accounts receivable	0
Allowance for doubtful accounts
Changes in allowance for doubtful accounts
Beginning balance	43,000,000	46,000,000	42,000,000
Additions	64,000,000	65,000,000	74,000,000
Deductions	(69,000,000)	(68,000,000)	(70,000,000)
Ending balance	$ 38,000,000	$ 43,000,000	$ 46,000,000