Accumulated Other Comprehensive Loss - Summary of Reclassifications Out of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income before income taxes	$ (1.4)	$ 25.3	$ 16.5	$ 35.9
Tax benefit	(1.8)	(8.7)	(9.5)	(11.5)
Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income before income taxes	1.3	1.8	1.8	3.4
Tax benefit	(0.4)	(0.4)	(0.4)	(1.0)
Net income (loss)	0.9	1.4	1.4	2.4
Reclassification out of accumulated other comprehensive income | Actuarial losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Actuarial losses	1.3	1.7	1.5	3.3
Reclassification out of accumulated other comprehensive income | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Actuarial losses	0.0	0.1	0.0	0.1
Reclassification out of accumulated other comprehensive income | Curtailment
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Actuarial losses	$ 0.0	$ 0.0	$ 0.3	$ 0.0