Quarterly Financial Information (Unaudited) Quarterly Financial Information (Unaudited) - Adjustments (Details) - USD ($) $ / shares in Units, $ 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
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Income	$ 254	$ 199	$ 151	$ 363	$ 159	$ 127	$ 212	$ 394	$ 966	$ 892	$ 835
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
Adjustments For ASU 2016-09 | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Income			$ 24	$ 27
Basic earnings per common share (in dollars per share)			$ 0.03	$ 0.04
Diluted earnings per common share (in dollars per share)			$ 0.03	$ 0.03