Computation of Basic/Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ 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
Numerator:
Consolidated net income	$ 650	$ 260	$ 402	$ 500	$ (584)	$ 188	$ 243	$ 426	$ 1,813	$ 273	$ 966
Less: Distributed earnings to unvested share-based awards that participate in earnings									0	0	(2)
Less: Undistributed earnings allocated to unvested share-based awards that participate in earnings									0	0	(2)
Numerator for basic and diluted earnings per common share—income available to common shareholders									$ 1,813	$ 273	$ 962
Denominator:
Denominator for basic earnings per common share - weighted-average common shares outstanding (in shares)									762	754	740
Effect of dilutive stock options and awards under the treasury stock method									9	12	14
Denominator for diluted earnings per common share—weighted-average common shares outstanding plus dilutive common shares under the treasury stock method									771	766	754
Basic earnings per common share (in dollars per share)	$ 0.85	$ 0.34	$ 0.53	$ 0.66	$ (0.77)	$ 0.25	$ 0.32	$ 0.57	$ 2.38	$ 0.36	$ 1.30
Diluted earnings per common share (in dollars per share)	$ 0.84	$ 0.34	$ 0.52	$ 0.65	$ (0.77)	$ 0.25	$ 0.32	$ 0.56	$ 2.35	$ 0.36	$ 1.28