Marketable Securities (Narratives) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2015 USD ($) CreditRatingAgency	Dec. 31, 2014 USD ($)
Marketable Securities
Gross unrealized losses (pre-tax) on marketable securities	$ 0	$ 1
Gross unrealized gains (pre-tax) on marketable securities	$ 0	$ 1
Percentage of asset-backed securities rated AAA/A-1+, Aaa/P-1, or AAA/F1+	83.20%
Estimated fair value of current plus long term asset backed securities	$ 107
Number of rating agencies for which asset backed securities must be rated | CreditRatingAgency	2
Number of rating agencies for asset backed securities that must be either Moody's or Standard and Poor's | CreditRatingAgency	1