Earnings (Loss) Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Basic EPS:
Net income (loss)	$ (2,241)	$ 185
Weighted average common shares outstanding (in shares)	425,713	451,951
Basic EPS (in dollars per share)	$ (5.26)	$ 0.41
Diluted EPS:
Net income (loss) for purposes of computing diluted EPS	$ (2,241)	$ 185
Share computation for diluted EPS:
Basic weighted average common shares outstanding (in shares)	425,713	451,951
Dilutive effect of stock awards (in shares)	0	1,478
Diluted weighted average common shares outstanding (in shares)	425,713	453,429
Diluted EPS (in dollars per share)	$ (5.26)	$ 0.41
Restricted stock unit awards excluded from the calculation of diluted EPS because inclusion would be antidilutive (in shares)	4,934	2,267