Defined Benefit and Postretirement Benefits (Components Of Net Periodic Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.6	$ 1.6	$ 4.7	$ 4.8
Interest cost	5.2	6.6	15.7	19.9
Expected return on plan assets	(14.8)	(14.7)	(44.7)	(43.5)
Recognized curtailment gain			(0.6)	0.0
Amortization of prior service cost	(0.1)	(0.1)	(0.2)	(0.2)
Amortization of net loss (gain)	6.5	4.8	19.6	14.5
Net periodic cost	(1.6)	(1.8)	(5.5)	(4.5)
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.1	0.3	0.2
Interest cost	0.2	0.2	0.5	0.6
Expected return on plan assets	0.0	0.0	0.0	0.0
Recognized curtailment gain			0.0	0.0
Amortization of prior service cost	(0.3)	(0.3)	(0.8)	(0.8)
Amortization of net loss (gain)	0.0	0.0	0.0	0.0
Net periodic cost	$ 0.0	$ 0.0	$ 0.0	$ 0.0