Earnings Per Common Share (Summary Of Calculation Of Basic And Diluted Earnings 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 Common Share [Abstract]
Net (loss) income used for basic and diluted calculation	$ (1,068.1)	$ 459.8	$ (510.5)	$ 1,047.7
Shares used in computing earnings per common share—basic	220.6	224.2	221.1	224.0
Stock awards	0.0	1.4	0.0	1.7
Shares used in computing earnings per common share-diluted	220.6	225.6	221.1	225.7
Earnings (loss) Per Share, Basic	$ (4.84)	$ 2.05	$ (2.31)	$ 4.68
Earnings (loss) Per Share, Diluted	$ (4.84)	$ 2.04	$ (2.31)	$ 4.64