Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Service cost	$ 3.3	$ 2.6	$ 4.1
Interest cost	2.9	7.3	11.5
Expected return on plan assets	(0.8)	(3.9)	(7.6)
Net actuarial loss (gain)	6.8	(4.1)	5.2
Net periodic benefit expense	$ 12.2	$ 1.9	$ 13.2