Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Retirement Benefits [Abstract]
Service cost	$ 1.0	$ 2.9	$ 3.1	$ 8.8
Interest cost	3.0	4.1	9.0	12.3
Expected return on plan assets	(2.2)	(2.9)	(6.7)	(8.7)
Actuarial loss	2.2	0.0	2.2	0.0
Net periodic benefit cost	$ 4.0	$ 4.1	$ 7.6	$ 12.4