Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic EPS Calculation
Earnings available to common stockholders - Basic	$ 9,195	$ (497)	$ 4,587
Weighted-average number of common shares outstanding (in shares)	1,882	1,872	1,867
Add: Deferred awards held as stock units (in shares)	1	1	1
Total weighted-average number of common shares outstanding (in shares)	1,883	1,873	1,868
Earnings per share of common stock - Basic (in dollars per share)	$ 4.88	$ (0.27)	$ 2.46
Diluted EPS Calculation
Earnings available to common stockholders - Diluted	$ 9,195	$ (497)	$ 4,587
Weighted-average number of common shares outstanding (in shares)	1,882	1,872	1,867
Add: Deferred awards held as stock units (in shares)	1	1	1
Add: Dilutive effect of employee stock-based awards (in shares)	15	0	7
Total weighted-average number of common shares outstanding (in shares)	1,898	1,873	1,875
Earnings per share of common stock - Diluted (in dollars per share)	$ 4.85	$ (0.27)	$ 2.45
Employee-based Awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of EPS	10