Net (Loss) Income Per Common Share (Basic And Diluted Net Income Per Share) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Net (loss) income		$ (65)	$ 339
Income allocated to participating securities		0	2
Net (loss) income attributable to common shares		$ (65)	$ 337
Basic average shares		141.0	158.0
Basic net (loss) income per common share		$ (0.46)	$ 2.14
Basic average shares		141.0	158.0
Net effect of the assumed exercise of stock options		0.0	2.0
Diluted average shares		141.0	160.0
Diluted net (loss) income per common share		$ (0.46)	$ 2.11
Stock awards excluded because exercise prices were greater than average market price of shares		1.2	0.4
Stock awards excluded due to net loss	[1]	2.6	0.0

[1] Average exercise price less than average market price of common shares for period; however, anti-dilutive due to net loss during the period.