(Loss) Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Loss income (Numerator):
Net (loss) income	$ (2,412)	$ 272	$ 292	$ 115	$ 1,117	$ 92	$ 17	$ 163	$ (1,733)	$ 1,389	$ 626
Net (loss) income applicable to common stock for computing basic earnings per common share									(1,733)	1,389	626
Net (loss) income as adjusted for purposes of computing diluted earnings per common share									$ (1,733)	$ 1,389	$ 626
Number of shares of common stock excluded from the computation of diluted earnings per share									2,700	4,700	3,300
Weighted average number of shares:
Outstanding during period (in shares)									1,078,409	635,576	545,946
Non-vested restricted stock (in shares)									(12,543)	(7,768)	(6,397)
Weighted average shares outstanding for computing basic earnings per common share (in shares)									1,065,866	627,808	539,549
Incremental common shares attributable to dilutive securities:
Shares issuable under convertible securities (in shares)									0	10	10
Shares issuable under incentive compensation plans (in shares)									0	875	1,120
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)									1,065,866	628,693	540,679
Basic (loss) earnings per common share (in dollars per share)	$ (2.26)	$ 0.25	$ 0.27	$ 0.11	$ 1.26	$ 0.17	$ 0.03	$ 0.30	$ (1.63)	$ 2.21	$ 1.16
Diluted (loss) earnings per common share (in dollars per share)	$ (2.26)	$ 0.25	$ 0.27	$ 0.11	$ 1.26	$ 0.17	$ 0.03	$ 0.30	$ (1.63)	$ 2.21	$ 1.16
Stock compensation plan
Loss income (Numerator):
Number of shares of common stock excluded from the computation of diluted earnings per share									4,600