Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income (Numerator):
Net income	$ 292	$ 17	$ 407	$ 180
Weighted-average number of shares:
Weighted average shares outstanding for computing basic earnings per common share (in shares)	1,064,711,000	541,361,000	1,064,663,000	540,909,000
Incremental common shares attributable to dilutive securities:
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	1,068,819,000	542,151,000	1,068,414,000	541,836,000
Basic earnings per common share (in dollars per share)	$ 0.27	$ 0.03	$ 0.38	$ 0.33
Diluted earnings per common share (in dollars per share)	$ 0.27	$ 0.03	$ 0.38	$ 0.33
Common Class A
Income (Numerator):
Net income	$ 292	$ 17	$ 407	$ 180
Earnings applicable to non-vested restricted stock	0	0	0	0
Net income applicable to common stock for computing basic earnings per common share	292	17	407	180
Net income as adjusted for purposes of computing diluted earnings per common share	$ 292	$ 17	$ 407	$ 180
Weighted-average number of shares:
Outstanding during period (in shares)	1,078,986,000	549,100,000	1,076,273,000	548,359,000
Non-vested restricted stock (in shares)	(14,275,000)	(7,739,000)	(11,610,000)	(7,450,000)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	1,064,711,000	541,361,000	1,064,663,000	540,909,000
Incremental common shares attributable to dilutive securities:
Shares issuable under convertible securities (in shares)	10,000	10,000	10,000	10,000
Shares issuable under incentive compensation plans (in shares)	4,098,000	780,000	3,741,000	917,000
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	1,068,819,000	542,151,000	1,068,414,000	541,836,000
Basic earnings per common share (in dollars per share)	$ 0.27	$ 0.03	$ 0.38	$ 0.33
Diluted earnings per common share (in dollars per share)	$ 0.27	$ 0.03	$ 0.38	$ 0.33
Number of shares of common stock excluded from the computation of diluted earnings per share (in shares)	3,100,000	3,100,000	3,700,000	3,700,000