Employee Benefit Plans - Components of Net Periodic Defined Benefit Pension Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3.0	$ 2.5	$ 8.5	$ 7.5	$ 9.9
Interest cost	4.1	4.5	11.8	13.4
Expected return on plan assets	(3.3)	(4.6)	(9.7)	(13.8)
Amortization of unrecognized prior service cost		0.1	0.2	0.2
Recognized actuarial losses	3.9	3.3	11.2	10.2
Total	7.7	5.8	22.0	17.5	$ 22.9
OPEB
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.1	0.1	0.1
Interest cost	0.1	0.2	0.3	0.4
Amortization of unrecognized prior service cost	(0.3)	(0.5)	(0.8)	(1.4)
Recognized actuarial losses	(0.1)	(0.1)	(0.2)	(0.1)
Total	$ (0.3)	$ (0.3)	$ (0.6)	$ (1.0)