Computation of Basic/Diluted Earnings Per Common Share - Computation EPS (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerator:
Consolidated net income	$ 127	$ 212	$ 463	$ 606
Less: Distributed earnings to unvested stock-based awards that participate in earnings	0	0	(2)	(4)
Less: Undistributed earnings allocated to unvested stock-based awards that participate in earnings	(1)	(2)	(1)	(5)
Numerator for basic and diluted earnings per common share — income available to common shareholders	$ 126	$ 210	$ 460	$ 597
Denominator:
Denominator for basic earnings per common share - weighted-average common shares outstanding (in shares)	739	727	737	725
Employee stock options and awards (in shares)	11	8	11	9
Denominator for diluted earnings per common share - weighted-average common shares outstanding plus dilutive common shares under treasury stock method (in shares)	750	735	748	734
Basic earnings (loss) per common share (in dollars per share)	$ 0.17	$ 0.29	$ 0.62	$ 0.82
Diluted earnings (loss) per common share (in dollars per share)	$ 0.17	$ 0.29	$ 0.61	$ 0.81