Marketable Securities (Narratives) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014 CreditRatingAgency	Dec. 31, 2013
Marketable Securities
Gross unrealized losses (pre-tax) on marketable securities	$ 2	$ 5
Gross unrealized gains (pre-tax) on marketable securities	1	1
Percentage of interests in auction rate securities to portfolio, maximum	1.00%
Estimated fair value of auction-rate securities	12	11
Gross unrealized losses (pre-tax) within accumulated other comprehensive income related to auction rate securities	1	2
Minimum period that auction rate securities have been in a loss position	12 months
Pre-determined intervals during which liquidity for auction rate securities is typically provided 1	7 days
Pre-determined intervals during which liquidity for auction rate securities is typically provided 2	28 days
Pre-determined intervals during which liquidity for auction rate securities is typically provided 3	35 days
Pre-determined intervals during which liquidity for auction rate securities is typically provided 4	90 days
Percentage of asset-backed securities rated AAA/A-1+, Aaa/P-1, or AAA/F1+	96.00%
Estimated fair value of current plus long term asset backed securities	$ 712
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