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	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Recognition of actuarial (gains) losses		$ 3.8	$ (0.5)	$ 1.9
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Recognition of actuarial (gains) losses		(63.1)	40.6
Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Recognition of actuarial (gains) losses	$ 0.9	(0.2)	(1.4)
Other Accumulated Other Comprehensive Income | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Beginning Balance		67.2	73.4
Recognition of actuarial (gains) losses		(3.9)	(0.9)
Current year actuarial loss (gain)		8.5	(5.3)
Ending Balance		71.8	67.2	$ 73.4
Total at December 31, 2018		71.8
Other Accumulated Other Comprehensive Income | Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Recognition of actuarial (gains) losses		0.1	1.4
Current year actuarial loss (gain)		(0.4)	(1.4)
Ending Balance		$ (0.3)
Beginning Balance			(5.1)
Amortization			5.1
Total at December 31, 2018			$ (0.3)