Stockholders' Equity (Common Stock Repurchase Program) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Oct. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Repurchase Program, Authorized Amount		$ 2,000
Repurchase of common stock shares	(2.4)	(6.5)	(6.3)
Average repurchase price (in dollars per share)	$ 19.08	$ 17.89	$ 19.57
Repurchase of common stock, value		$ 116	$ 124
Payments for Repurchase of Common Stock	$ 45	116	$ 124
Remaining amount authorized for repurchase		$ 600
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Repurchase of common stock shares		(6.5)	(6.3)
Repurchase of common stock, value		$ 0	$ 1
Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Repurchase Program, Authorized Amount				$ 1,000