Earnings (Loss) per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share Reconciliation [Abstract]
Net income (loss)	$ (40)	$ 557	$ 560	$ 1,353
Participating share-based awards — allocated income	(1)	(4)	(3)	(10)
Net income (loss) available to common shareholders -- basic	(41)	553	557	1,343
Net income (loss) available to common shareholders -- diluted	$ (41)	$ 553	$ 557	$ 1,343
Weighted average shares outstanding, basic (in shares)	251.6	261.3	253.6	262.1
Weighted average effects of dilutive securities - stock options and performance shares (in shares)	0.0	2.4	1.1	2.1
Weighted average shares outstanding, diluted (in shares)	251.6	263.7	254.7	264.2
Net income (loss) per common share, basic (in dollars per share)	$ (0.16)	$ 2.11	$ 2.19	$ 5.12
Net income (loss) per common share, diluted (in dollars per share)	$ (0.16)	$ 2.10	$ 2.19	$ 5.08
Excluded allocation of undistributed loss to participating share-based awards because such allocation would result in anti-dilution of basic and diluted earnings per share	$ 2
Amount of shares excluded from computation of net loss per diluted common share since their effects on income was anti-dilutive	0.7