Shareholders' Equity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock
Number of shares of common stock authorized	550,000,000
Number of shares of preferred stock authorized	15	15,000,000
Share repurchases, net of issuances	$ 59	$ 100	$ 360
Series A Junior Participating Preferred Stock
Class of Stock
Number of shares of preferred stock authorized	550,000