Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income (Numerator):
Net income	$ 1,117	$ 92	$ 17	$ 163	$ 42	$ 152	$ 196	$ 236	$ 1,389	$ 626	$ 878
Earnings applicable to non-vested restricted stock									0	0	0
Net income applicable to common stock for computing basic earnings per common share									1,389	626	878
Net income as adjusted for purposes of computing diluted earnings per common share									$ 1,389	$ 626	$ 878
Weighted average number of shares:
Outstanding during period (in shares)									635,576	545,946	559,260
Non-vested restricted stock (in shares)									(7,768)	(6,397)	(4,982)
Weighted average shares outstanding for computing basic earnings per common share (in shares)									627,808	539,549	554,278
Incremental common shares attributable to dilutive securities:
Shares issuable under convertible securities (in shares)									10	10	10
Shares issuable under incentive compensation plans (in shares)									875	1,120	805
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)									628,693	540,679	555,093
Basic earnings per common share (in dollars per share)	$ 1.26	$ 0.17	$ 0.03	$ 0.30	$ 0.08	$ 0.28	$ 0.36	$ 0.44	$ 2.21	$ 1.16	$ 1.58
Diluted earnings per common share (in dollars per share)	$ 1.26	$ 0.17	$ 0.03	$ 0.30	$ 0.08	$ 0.28	$ 0.36	$ 0.44	$ 2.21	$ 1.16	$ 1.58
Number of shares of common stock excluded from the computation of diluted earnings per share									4,700	3,300	3,100