Summary of Significant Accounting Policies Significant Accounting Policies - Allowance for Doubtful Accounts (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	$ 9	$ 9	$ 8
Charged to operating costs and expenses, net	13	14	13
Write-offs, net	(14)	(14)	(12)
Ending balance	$ 8	$ 9	$ 9