Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ 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
Earnings Per Share [Abstract]
Net income	$ 980	$ 1,315	$ 1,485	$ 746	$ (712)	$ 357	$ 801	$ 213	$ 4,526	$ 659	$ 10,540
Basic weighted average shares outstanding (shares)									797	836	849
Dilutive effect of share-based awards (shares)									7	9	9
Diluted weighted average shares outstanding (shares)									804	845	858
Basic earnings per share (usd per share)	$ 1.26	$ 1.67	$ 1.85	$ 0.91	$ (0.86)	$ 0.43	$ 0.95	$ 0.25	$ 5.68	$ 0.79	$ 12.41
Diluted earnings per share (usd per share)	$ 1.25	$ 1.65	$ 1.83	$ 0.90	$ (0.86)	$ 0.42	$ 0.94	$ 0.25	$ 5.63	$ 0.78	$ 12.29