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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income—basic and diluted	$ (452)	$ 553	$ 217	$ (31)	$ 642	$ 102	$ 100	$ 201	$ 287	$ 1,045	$ 591
Denominator:
Basic weighted-average common shares outstanding									163.2	164.2	164.5
Diluted weighted-average shares									164.5	166.0	166.5
Earnings per share:
Basic (in usd per share)	$ (2.78)	$ 3.40	$ 1.33	$ (0.19)	$ 3.92	$ 0.62	$ 0.61	$ 1.22	$ 1.76	$ 6.36	$ 3.59
Diluted (in usd per share)	$ (2.78)	$ 3.38	$ 1.32	$ (0.19)	$ 3.88	$ 0.61	$ 0.60	$ 1.21	$ 1.75	$ 6.29	$ 3.55
Number of anti-dilutive share-based awards									1.3	0.7	0.3
Stock Options
Denominator:
Effect of stock-based awards									0.8	1.1	1.1
Restricted Stock Units (RSUs)
Denominator:
Effect of stock-based awards									0.5	0.7	0.9