Cash, Cash Equivalents, and Marketable Securities Cash, Cash Equivalents, and Marketable Securities - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Investments, Warrant Assets
Gain (loss) on Equity Warrant Assets	$ 9	$ (1)	$ (11)	$ 1
Level 2 Securities
Investments, Warrant Assets
Equity Warrant Assets	$ 142		$ 142		$ 16