Loss Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of shares of common stock excluded from the computation of diluted earnings per share	3,200	6,800	2,700
Loss (Numerator):
Net loss	$ (1,232)	$ (5,269)	$ (1,733)
Net loss applicable to common stock for computing basic earnings per common share	(1,232)	(5,269)	(1,733)
Net loss as adjusted for purposes of computing diluted earnings per common share	$ (1,232)	$ (5,269)	$ (1,733)
Weighted average number of shares:
Outstanding during period (in shares)	1,096,284	1,088,730	1,078,409
Non-vested restricted stock (in shares)	(17,154)	(17,289)	(12,543)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	1,079,130	1,071,441	1,065,866
Incremental common shares attributable to dilutive securities:
Shares issuable under convertible securities (in shares)	0	0	0
Shares issuable under incentive compensation plans (in shares)	0	0	0
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	1,079,130	1,071,441	1,065,866
Basic loss per common share (in dollars per share)	$ (1.14)	$ (4.92)	$ (1.63)
Diluted loss per common share (in dollars per share)	$ (1.14)	$ (4.92)	$ (1.63)
Stock compensation plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of shares of common stock excluded from the computation of diluted earnings per share	5,300	3,000	4,600