Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Retirement Benefits [Abstract]
Service cost	$ 0.7	$ 0.8	$ 2.1	$ 2.4
Interest cost	0.5	0.7	1.5	2.1
Expected return on plan assets	(0.1)	(0.2)	(0.3)	(0.6)
Net periodic benefit cost	$ 1.1	$ 1.3	$ 3.3	$ 3.9