Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Allowance for Doubtful Accounts Rollforward) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance of allowance for doubtful accounts	$ 1	$ 1	$ 1	$ 3
Bad debt expense	1	1	(1)
Charge-offs	(1)	(1)	(1)
Ending balance of allowance for doubtful accounts	$ 1	$ 1	$ 1	$ 3