Common Stock (Schedule of Intrinsic Value of Options Exercised) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Equity and Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Intrinsic value of options outstanding as of December 31	$ 29	$ 53
Intrinsic value of options exercisable as of December 31	28	48
Intrinsic value of options exercised	$ 7	$ 17	$ 21
Weighted-average grant date fair value per option (in dollars per share)	$ 0.00	$ 0.00	$ 24.56