Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 270	$ 285	$ 250	$ 247	$ 129	$ 147	$ 193	$ 145	$ 1,052	$ 614	$ 552
Preferred stock dividends									36	25	0
Net income available to common shareholders									$ 1,016	$ 589	$ 552
Share data (in thousands):
Basic weighted-average shares outstanding									260,600	273,190	277,789
Effect of weighted-average dilutive securities:
Restricted stock and options(1)									1,053	1,076	872
Convertible debentures									16	86	387
Diluted weighted-average shares outstanding(2)									261,669	274,352	279,048
Basic earnings per common share (in dollars per share)	$ 1.07	$ 1.01	$ 0.95	$ 0.88	$ 0.48	$ 0.49	$ 0.70	$ 0.48	$ 3.90	$ 2.16	$ 1.99
Diluted earnings per common share (in dollars per share)	$ 1.06	$ 1.00	$ 0.95	$ 0.88	$ 0.48	$ 0.49	$ 0.70	$ 0.48	$ 3.88	$ 2.15	$ 1.98