Derivative Instruments and Hedging Activities (Summary Of Accumulated Other Comprehensive Loss Related To Derivatives) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
After-Tax Amount
Total stockholders’ equity	$ 3,124.7	$ 2,690.6	$ 3,018.0	$ 2,907.0
Net losses reclassified from accumulated other comprehensive loss into income	7.6	3.4
Balance end of period	3,124.7	$ 2,690.6
Deferred Net Gains (Losses) on Derivatives
Before-Tax Amount
Accumulated derivative net gains (losses) as the beginning of the period	(3.0)
Net changes in fair value of derivatives	1.3
Net gains reclassified from accumulated other comprehensive loss into income	3.4
Accumulated derivative net gains (losses) as of the end of the period	1.7
Income Tax
Accumulated derivative net gains (losses) as of the beginning of the period	(0.5)
Net changes in fair value of derivatives	0.3
Income tax provision	(0.3)
Accumulated derivative net gains (losses) as of the end of the period	(0.5)
After-Tax Amount
Total stockholders’ equity	2.2		$ (2.5)
Net changes in fair value of derivatives	1.0
Net losses reclassified from accumulated other comprehensive loss into income	3.7
Balance end of period	$ 2.2