Accumulated Other Comprehensive Loss - Summary of Reclassifications Out of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income before income taxes	$ (28.6)	$ (9.3)	$ (12.1)	$ 26.6
Tax benefit	7.3	0.9	(2.2)	(10.6)
Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income before income taxes	1.5	1.8	3.3	5.2
Tax benefit	(0.5)	(0.5)	(0.9)	(1.5)
Net income (loss)	1.0	1.3	2.4	3.7
Reclassification out of accumulated other comprehensive income | Actuarial losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Actuarial losses	1.5	1.7	3.0	5.0
Reclassification out of accumulated other comprehensive income | Prior service costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Actuarial losses	0.0	0.1	0.0	0.2
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