Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 129	$ 147	$ 193	$ 145	$ 127	$ 139	$ 133	$ 153	$ 614	$ 552	$ 268
Preferred stock dividends									25	0	0
Net income available to common shareholders									$ 589	$ 552	$ 268
Share data (in thousands):
Basic weighted-average shares outstanding									273,190	277,789	290,762
Effect of weighted-average dilutive securities:
Restricted stock and options									1,076	872	1,429
Convertible debentures									86	387	2,820
Diluted weighted-average shares outstanding(1)									274,352	279,048	295,011
Basic earnings per common share (in dollars per share)	$ 0.48	$ 0.49	$ 0.70	$ 0.48	$ 0.46	$ 0.51	$ 0.48	$ 0.54	$ 2.16	$ 1.99	$ 0.92
Diluted earnings per common share (in dollars per share)	$ 0.48	$ 0.49	$ 0.70	$ 0.48	$ 0.46	$ 0.51	$ 0.48	$ 0.53	$ 2.15	$ 1.98	$ 0.91