Earnings Per Share (Schedule of Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net income (loss)	$ (880)	$ (1,547)	$ 138	$ 594	$ (2,427)	$ 732
Number of common shares:
Weighted average outstanding (in shares)	541,079,192		539,005,941		540,693,841	539,362,984
Issued upon assumed exercise of outstanding stock options (in shares)	0		0		0	0
Effect of issuance of non-vested restricted common stock (in shares)	0		311,732		0	481,948
Effect of issuance of non-vested performance units (in shares)	0		629,380		0	779,810
Weighted average and potential dilutive outstanding (in shares)	541,079,192		539,947,053		540,693,841	540,624,742
Earnings per common share
Basic (in dollars per share)	$ (1.63)		$ 0.26		$ (4.49)	$ 1.36
Diluted (in dollars per share)	$ (1.63)		$ 0.26		$ (4.49)	$ 1.35