Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Basic:
Net income	$ 193	$ 133	$ 338	$ 286
Preferred stock dividends	0	0	13	0
Net income available to common shareholders	$ 193	$ 133	$ 325	$ 286
Basic weighted-average shares outstanding (in thousands)	275,410	277,013	275,167	281,141
Basic earnings per share (in dollars per share)	$ 0.70	$ 0.48	$ 1.18	$ 1.02
Diluted:
Net income	$ 193	$ 133	$ 338	$ 286
Preferred stock dividends	0	0	13	0
Net income available to common shareholders	$ 193	$ 133	$ 325	$ 286
Basic weighted-average shares outstanding (in thousands)	275,410	277,013	275,167	281,141
Effect of dilutive securities:
Weighted-average restricted stock and options issued to employees (in thousands)	843	646	1,048	828
Weighted-average convertible debentures (in thousands)	19	319	155	457
Diluted weighted-average shares outstanding (in thousands)	276,272	277,978	276,370	282,426
Diluted earnings per share (in dollars per share)	$ 0.70	$ 0.48	$ 1.17	$ 1.01
Stock options and restricted stock awards and units | Maximum [Member]
Antidilutive securities excluded from calculation of earnings per share [Line Items]
Antidilutive securities excluded from the calculation of diluted earnings per share	100	100	100	100