Common Stock (Schedule of Intrinsic Value of Options Exercised) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Intrinsic value of options outstanding as of December 31	$ 80	$ 43
Intrinsic value of options exercisable as of December 31	72	43
Intrinsic value of options exercised	$ 6	$ 4	$ 7
Weighted-average grant date fair value per option (in dollars per share)	$ 84.60	$ 0.00	$ 0.00