Net Income Per Common Share (Basic And Diluted Net Income Per Share) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net (loss) income	$ 113	$ 298	$ 48	$ 637
Income allocated to participating securities	1	1	1	3
Net (loss) income attributable to common shares	$ 112	$ 297	$ 47	$ 634
Basic average shares	139.0	152.0	140.0	155.0
Basic net (loss) income per common share	$ 0.81	$ 1.95	$ 0.34	$ 4.10
Basic average shares	139.0	152.0	140.0	155.0
Net effect of the assumed exercise of stock options	0.0	1.0	1.0	1.0
Diluted average shares	139.0	153.0	141.0	156.0
Diluted net (loss) income per common share	$ 0.80	$ 1.94	$ 0.34	$ 4.06