Summary of Significant Accounting Policies - Allowance for Doubtful Accounts (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Allowance for Doubtful Accounts [Line Items]
Ending Balance	$ 14	$ 8	$ 8
Allowance for Doubtful Accounts [Member]
Allowance for Doubtful Accounts [Line Items]
Beginning Balance	8	8	7
Provision for Doubtful Accounts	14	15	15
Allowance for Doubtful Accounts Receivable, Write-offs	(8)	(15)	(14)
Ending Balance		$ 8	$ 8