Earnings per Common Share (Schedule of Basic and Diluted Earnings per Common Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income from continuing operations	$ 374	$ 280	$ 974	$ 747
Income (loss) from discontinued operations, net of tax	0	2	(1)	1
Net income attributable to common stockholders	$ 374	$ 282	$ 973	$ 748
Basic weighted-average common shares outstanding	422,187	449,169	429,625	457,612
Diluted weighted-average common shares outstanding	424,784	451,078	432,038	458,848
Earnings Per Share, Basic [Abstract]
Net income from continuing operations, basic earnings per share	$ 0.89	$ 0.62	$ 2.27	$ 1.63
Loss from discontinued operations, net of tax, basic earnings per share	0.00	0.00	0.00	0.00
Net income, basic earnings per share	0.89	0.63	2.26	1.63
Earnings Per Share, Diluted [Abstract]
Net income from continuing operations, diluted earnings per share	0.88	0.62	2.25	1.63
Loss from discontinued operations, net of tax, diluted earnings per share	0.00	0.00	0.00	0.00
Net income, diluted earnings per share	$ 0.88	$ 0.63	$ 2.25	$ 1.63