Net Income Per Common Share (Calculations For Basic And Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income—basic and diluted	$ 642	$ 102	$ 100	$ 201	$ 100	$ 180	$ 123	$ 188	$ 1,045	$ 591	$ 634
Denominator:
Basic weighted-average common shares outstanding									164.2	164.5	164.0
Diluted weighted-average shares									166.0	166.5	165.6
Earnings per share:
Basic (in usd per share)	$ 3.92	$ 0.62	$ 0.61	$ 1.22	$ 0.61	$ 1.09	$ 0.75	$ 1.14	$ 6.36	$ 3.59	$ 3.87
Diluted (in usd per share)	$ 3.88	$ 0.61	$ 0.60	$ 1.21	$ 0.60	$ 1.08	$ 0.74	$ 1.13	$ 6.29	$ 3.55	$ 3.83
Number of anti-dilutive share-based awards									0.7	0.3	0.4
Stock Options
Denominator:
Effect of stock-based awards									1.1	1.1	1.0
Restricted Stock Units (RSUs)
Denominator:
Effect of stock-based awards									0.7	0.9	0.6