Computation of Basic/Diluted Earnings Per Common Share - Computation EPS (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Consolidated net income	$ 188	$ 199	$ 858	$ 713
Less: Distributed earnings to unvested share-based awards that participate in earnings	0	0	0	(2)
Less: Undistributed earnings allocated to unvested share-based awards that participate in earnings	0	(1)	0	(2)
Numerator for basic and diluted earnings per common share—income available to common shareholders	$ 188	$ 198	$ 858	$ 709
Denominator:
Denominator for basic earnings per common share - weighted-average common shares outstanding (in shares)	755	742	753	739
Effect of potential dilutive common shares under the treasury stock method:
Employee stock options and awards (in shares)	11	14	11	14
Denominator for diluted earnings per common share - weighted-average common shares outstanding plus dilutive common shares under treasury stock method (in shares)	766	756	764	753
Basic earnings (loss) per common share (in dollars per share)	$ 0.25	$ 0.27	$ 1.14	$ 0.96
Diluted earnings (loss) per common share (in dollars per share)	$ 0.25	$ 0.26	$ 1.12	$ 0.94