Defined Benefit Plans - Amounts in Accumulated Other Comprehensive loss that have not yet been Recognized as Components of Net Periodic Benefit Cost (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 (gains) losses		$ 2.5	$ (0.5)	$ 1.9	$ 8.6
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Recognition of actuarial (gains) losses			40.6	11.7
Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Recognition of actuarial (gains) losses	$ 0.9		(1.4)	1.6
Other Accumulated Other Comprehensive Income | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Beginning Balance	71.1		73.4	71.1
Recognition of actuarial (gains) losses			(0.9)
Current year actuarial loss (gain)			(5.3)	2.3
Ending Balance			67.2	73.4	71.1
Beginning Balance	0.1			0.1
Prior service cost recognition due to curtailment				(0.1)
Ending Balance					0.1
Total at December 31, 2017			67.2
Other Accumulated Other Comprehensive Income | Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Beginning Balance	0.3			0.3
Recognition of actuarial (gains) losses			1.4	(1.9)
Current year actuarial loss (gain)			(1.4)	1.6
Ending Balance					0.3
Beginning Balance	$ (6.4)		(5.1)	(6.4)
Prior service cost recognition due to plan amendments				(12.2)
Amortization			$ 5.1	13.5
Ending Balance				$ (5.1)	$ (6.4)