Marketable Securities - Narrative (Details) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) item	Dec. 31, 2014 USD ($)
Marketable Securities
Gross unrealized losses (pre-tax) on marketable securities		$ 1
Gross unrealized gains (pre-tax) on marketable securities		$ 1
Percentage of asset-backed securities rated AAA/A-1+, Aaa/P-1, or AAA/F1+	80.50%
Estimated fair value of current plus long term asset backed securities	$ 70
Number of rating agencies for which asset backed securities must be rated | item	2
Number of rating agencies for asset backed securities that must be either Moody's or Standard and Poor's | item	1