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, 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
Numerator:
Consolidated net income	$ 525	$ 204	$ 328	$ 447	$ 685	$ 260	$ 402	$ 500	$ 1,503	$ 1,848	$ 273
Denominator:
Denominator for basic earnings per common share - weighted-average common shares outstanding (in shares)									767	762	754
Effect of dilutive stock options and awards under the treasury stock method									4	9	12
Denominator for diluted earnings per common share—weighted-average common shares outstanding plus dilutive common shares under the treasury stock method									771	771	766
Basic earnings per common share (in dollars per share)	$ 0.68	$ 0.27	$ 0.43	$ 0.58	$ 0.90	$ 0.34	$ 0.53	$ 0.66	$ 1.96	$ 2.43	$ 0.36
Diluted earnings per common share (in dollars per share)	$ 0.68	$ 0.26	$ 0.43	$ 0.58	$ 0.89	$ 0.34	$ 0.52	$ 0.65	$ 1.95	$ 2.40	$ 0.36