Employee Benefit Plans - Components of Net Periodic Defined Benefit Pension Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2.9	$ 2.8	$ 5.9	$ 5.5
Interest cost	4.0	3.9	8.0	7.7
Expected return on plan assets	(3.8)	(3.2)	(7.8)	(6.4)
Amortization of unrecognized prior service cost		0.1	0.1	0.2
Recognized actuarial losses	3.9	3.7	7.7	7.3
Total	7.0	7.3	13.9	14.3
OPEB
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.1	0.1	0.1
Interest cost		0.1	0.1	0.2
Amortization of unrecognized prior service cost	(0.2)	(0.3)	(0.5)	(0.5)
Recognized actuarial losses	(0.1)		(0.1)	(0.1)
Total	$ (0.2)	$ (0.1)	$ (0.4)	$ (0.3)