Defined Benefit Plans - Components of Net Periodic Benefit Cost for Pension and Postretirement Benefits (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Recognition of actuarial losses (gains)		$ 2.5	$ (0.5)	$ 1.9	$ 8.6
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost			0.6	9.6
Interest cost			33.3	34.4
Recognition of actuarial losses (gains)			40.6	11.7
Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost				0.3
Recognition of actuarial losses (gains)	$ 0.9		(1.4)	1.6
Amortization of prior service cost (credits)	$ 10.7
Net Periodic Benefit Cost | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost			0.6	9.6	11.5
Interest cost			33.3	34.4	33.7
Expected return on plan assets			(37.3)	(37.2)	(40.2)
Recognition of actuarial losses (gains)			0.9		2.9
Amortization of prior service cost (credits)					0.1
Net periodic benefit (income) cost			(2.5)	6.8	8.0
Net Periodic Benefit Cost | Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost					0.1
Interest cost				0.3	0.6
Recognition of actuarial losses (gains)			(1.4)	1.9	(0.4)
Amortization of prior service cost (credits)			(5.1)	(13.5)	(13.5)
Net periodic benefit (income) cost			$ (6.5)	$ (11.3)	$ (13.2)