Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Basic:
Net income	$ 153	$ 40
Basic weighted-average shares outstanding (in thousands)	285,274	289,741
Basic earnings per share (in dollars per share)	$ 0.54	$ 0.14
Diluted:
Net income	$ 153	$ 40
Basic weighted-average shares outstanding (in thousands)	285,274	289,741
Effect of dilutive securities:
Weighted-average convertible debentures (in thousands)	597	3,640
Weighted-average options and restricted stock issued to employees (in thousands)	809	1,341
Diluted weighted-average shares outstanding (in thousands)	286,680	294,722
Diluted earnings per share (in dollars per share)	$ 0.53	$ 0.14
Stock options and restricted stock awards and units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of EPS, shares	100	200