Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Basic:
Net income	$ 89	$ (153)	$ 292	$ 40	$ 41	$ 86	$ 69	$ 97	$ 268	$ 293	$ 86
Basic weighted-average shares outstanding (in thousands)									290,762	288,705	286,991
Basic earnings per share (in dollars per share)	$ 0.31	$ (0.53)	$ 1.01	$ 0.14	$ 0.14	$ 0.30	$ 0.24	$ 0.34	$ 0.92	$ 1.02	$ 0.30
Diluted:
Net income	$ 89	$ (153)	$ 292	$ 40	$ 41	$ 86	$ 69	$ 97	$ 268	$ 293	$ 86
Basic weighted-average shares outstanding (in thousands)									290,762	288,705	286,991
Effect of dilutive securities:
Weighted-average convertible debentures (in thousands)									2,820	3,999	4,125
Weighted-average options and restricted stock issued to employees (in thousands)									1,429	1,399	1,473
Diluted weighted-average shares outstanding (in thousands)									295,011	294,103	292,589
Diluted earnings per share (in dollars per share)	$ 0.30	$ (0.53)	$ 0.99	$ 0.14	$ 0.14	$ 0.29	$ 0.24	$ 0.33	$ 0.91	$ 1.00	$ 0.29
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 Earnings Per Share, Amount									100	500	1,700