Restructuring charges - Summary of Activity Related to Accrued Liabilities Associated with Redundancy and Restructuring (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Restructuring Reserve [Roll Forward]
Beginning balance	$ 5.8	$ 9.4
Charge to earnings	9.7	2.6
Cash paid	(11.4)	(6.2)
Ending balance	4.1	5.8
Employee termination costs
Restructuring Reserve [Roll Forward]
Beginning balance	3.7	4.2
Charge to earnings	9.7	2.5
Cash paid	(10.8)	(3.0)
Ending balance	2.6	3.7
Facility exit costs
Restructuring Reserve [Roll Forward]
Beginning balance	1.9	5.0
Charge to earnings	0.0	0.1
Cash paid	(0.6)	(3.2)
Ending balance	1.3	1.9
Other exit costs
Restructuring Reserve [Roll Forward]
Beginning balance	0.2	0.2
Charge to earnings	0.0	0.0
Cash paid	0.0	0.0
Ending balance	$ 0.2	$ 0.2