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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Consolidated net income	$ 254	$ 199	$ 151	$ 363	$ 159	$ 127	$ 212	$ 394	$ 966	$ 892	$ 835
Less: Distributed earnings to unvested share-based awards that participate in earnings									(2)	(4)	(4)
Less: Undistributed earnings allocated to unvested share-based awards that participate in earnings									(2)	(7)	(14)
Numerator for basic and diluted earnings per common share—income available to common shareholders									$ 962	$ 881	$ 817
Denominator:
Denominator for basic earnings per common share - weighted-average common shares outstanding (in shares)									740	728	716
Employee stock options and awards									14	11	10
Denominator for diluted earnings per common share—weighted-average common shares outstanding plus dilutive common shares under the treasury stock method									754	739	726
Basic earnings per common share (in dollars per share)	$ 0.34	$ 0.27	$ 0.20	$ 0.49	$ 0.22	$ 0.17	$ 0.29	$ 0.54	$ 1.30	$ 1.21	$ 1.14
Diluted earnings per common share (in dollars per share)	$ 0.33	$ 0.26	$ 0.20	$ 0.48	$ 0.21	$ 0.17	$ 0.29	$ 0.53	$ 1.28	$ 1.19	$ 1.13