Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Retirement Benefits [Abstract]
Service cost	$ 0.7	$ 0.8
Interest cost	0.5	0.7
Expected return on plan assets	(0.1)	(0.2)
Net periodic benefit cost	$ 1.1	$ 1.3