Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Service cost	$ 2.6	$ 4.1	$ 11.7
Interest cost	7.3	11.5	16.4
Expected return on plan assets	(3.9)	(7.6)	(11.6)
Net actuarial (gain) loss	(4.1)	5.2	8.4
Net periodic benefit expense	$ 1.9	$ 13.2	$ 24.9