Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Retirement Benefits [Abstract]
Service cost	$ 0.8	$ 0.7	$ 2.4	$ 1.8
Interest cost	0.7	0.9	2.1	6.3
Expected return on plan assets	(0.2)	(0.2)	(0.6)	(3.6)
Pension settlement gain	0.0	0.4	0.0	(11.8)
Net periodic benefit cost	$ 1.3	$ 1.8	$ 3.9	$ (7.3)