(Loss) Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Loss income (Numerator):
Net (loss) income	$ 223	$ 302	$ 371	$ (6,165)	$ (2,412)	$ 272	$ 292	$ 115	$ (5,269)	$ (1,733)	$ 1,389
Net (loss) income applicable to common stock for computing basic earnings per common share									(5,269)	(1,733)	1,389
Net (loss) income as adjusted for purposes of computing diluted earnings per common share									$ (5,269)	$ (1,733)	$ 1,389
Number of shares of common stock excluded from the computation of diluted earnings per share									6,800	2,700	4,700
Weighted average number of shares:
Outstanding during period (in shares)									1,088,730	1,078,409	635,576
Non-vested restricted stock (in shares)									(17,289)	(12,543)	(7,768)
Weighted average shares outstanding for computing basic earnings per common share (in shares)									1,071,441	1,065,866	627,808
Incremental common shares attributable to dilutive securities:
Shares issuable under convertible securities (in shares)									0	0	10
Shares issuable under incentive compensation plans (in shares)									0	0	875
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)									1,071,441	1,065,866	628,693
Basic (loss) earnings per common share (in dollars per share)	$ 0.21	$ 0.28	$ 0.35	$ (5.77)	$ (2.26)	$ 0.25	$ 0.27	$ 0.11	$ (4.92)	$ (1.63)	$ 2.21
Diluted (loss) earnings per common share (in dollars per share)	$ 0.21	$ 0.28	$ 0.35	$ (5.77)	$ (2.26)	$ 0.25	$ 0.27	$ 0.11	$ (4.92)	$ (1.63)	$ 2.21
Stock compensation plan
Loss income (Numerator):
Number of shares of common stock excluded from the computation of diluted earnings per share									3,000	4,600