Accounts Receivable (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other receivables	$ 18	$ 22
Total accounts receivable	2,095	2,150
Less: allowance for doubtful accounts	(152)	(162)
Accounts receivable, less allowance	1,943	1,988
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	162	155	$ 158
Additions	177	159	152
Deductions	(187)	(152)	(155)
Ending balance	152	162	$ 155
Earned and unbilled receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	288	307
Trade and purchased receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	$ 1,789	$ 1,821