Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Retirement Benefits [Abstract]
Service cost	$ 0.7	$ 1.0	$ 1.4	$ 2.1
Interest cost	2.7	3.0	5.4	6.0
Expected return on plan assets	(1.7)	(2.2)	(3.4)	(4.5)
Actuarial gain	(12.2)	0.0	(12.2)	0.0
Net periodic benefit cost	$ (10.5)	$ 1.8	$ (8.8)	$ 3.6