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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Consolidated net income	$ 159	$ 127	$ 212	$ 394	$ 361	$ (23)	$ 204	$ 293	$ 892	$ 835	$ 1,010
Less: Distributed earnings to unvested share-based awards that participate in earnings									(4)	(4)	(5)
Less: Undistributed earnings allocated to unvested stock-based awards that participate in earnings									(7)	(14)	(18)
Numerator for basic and diluted earnings per common share—income available to common shareholders									$ 881	$ 817	$ 987
Denominator:
Denominator for basic earnings per common share - weighted-average common shares outstanding (in shares)									728	716	1,024
Effect of potential dilutive common shares under the treasury stock method: Employee stock options (in shares)									11	10	11
Denominator for diluted earnings per common share - weighted-average common shares outstanding plus dilutive effect of employee stock options and others (in shares)									739	726	1,035
Basic earnings per common share (in dollars per share)	$ 0.22	$ 0.17	$ 0.29	$ 0.54	$ 0.49	$ (0.03)	$ 0.28	$ 0.40	$ 1.21	$ 1.14	$ 0.96
Diluted earnings per common share (in dollars per share)	$ 0.21	$ 0.17	$ 0.29	$ 0.53	$ 0.49	$ (0.03)	$ 0.28	$ 0.40	$ 1.19	$ 1.13	$ 0.95