Earnings (Loss) Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Basic EPS:
Net income (loss)	$ (2,178)	$ (2,399)	$ (2,067)	$ (2,241)	$ 414	$ 425	$ 662	$ 185	$ (8,885)	$ 1,686	$ 1,412
Weighted average common shares outstanding (in shares)					434,578	441,915	445,008	451,951	483,888	443,363	464,236
Basic EPS (in dollars per share)					$ 0.95	$ 0.96	$ 1.49	$ 0.41	$ (18.36)	$ 3.80	$ 3.04
Diluted EPS:
Net income (loss) for purposes of computing diluted EPS									$ (8,885)	$ 1,686	$ 1,412
Share computation for diluted EPS:
Basic weighted average common shares outstanding (in shares)					434,578	441,915	445,008	451,951	483,888	443,363	464,236
Dilutive effect of stock awards (in shares)									0	906	1,424
Diluted weighted average common shares outstanding (in shares)					435,659	442,401	445,587	453,429	483,888	444,269	465,660
Diluted EPS (in dollars per share)					$ 0.95	$ 0.96	$ 1.49	$ 0.41	$ (18.36)	$ 3.79	$ 3.03
6.50% convertible senior notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)									31,882	0	0
Restricted stock unit awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)									4,584	2,520	1,266
Issuance of PSP1 Warrants (see Note 1(b))
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)									349	0	0
Treasury Loan Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)									107	0	0