Marketable Securities (Narratives) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 CreditRatingAgency	Dec. 31, 2013
Marketable Securities
Gross unrealized losses (pre-tax) on marketable securities	$ 1	$ 5
Gross unrealized gains (pre-tax) on marketable securities	1	1
Percentage of asset-backed securities rated AAA/A-1+, Aaa/P-1, or AAA/F1+	96.60%
Estimated fair value of current plus long term asset backed securities	$ 538
Number of rating agencies for which asset backed securities must be rated	2
Number of rating agencies for asset backed securities that must be either Moody's or Standard and Poor's	1