Revenue - Disaggregation of Sales - Schedule of Disaggregates of Net Sales of our Chemicals Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation Of Revenue [Line Items]
Net sales	$ 458.6	$ 475.2	$ 1,332.9	$ 1,483.5
Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	416.9	437.4	1,223.9	1,358.4
Point of Origin | Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	416.9	437.4	1,223.9	1,358.4
Point of Origin | Chemicals | Germany
Disaggregation Of Revenue [Line Items]
Net sales	208.1	238.3	620.3	706.0
Point of Origin | Chemicals | United States
Disaggregation Of Revenue [Line Items]
Net sales	272.9	227.7	736.5	762.3
Point of Origin | Chemicals | Canada
Disaggregation Of Revenue [Line Items]
Net sales	93.8	84.0	247.1	258.4
Point of Origin | Chemicals | Belgium
Disaggregation Of Revenue [Line Items]
Net sales	64.9	58.3	181.9	205.7
Point of Origin | Chemicals | Norway
Disaggregation Of Revenue [Line Items]
Net sales	46.6	46.0	163.5	145.2
Point of Origin | Chemicals | Eliminations
Disaggregation Of Revenue [Line Items]
Net sales	(269.4)	(216.9)	(725.4)	(719.2)
Point of Destination | Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	416.9	437.4	1,223.9	1,358.4
Point of Destination | Chemicals | Europe
Disaggregation Of Revenue [Line Items]
Net sales	194.3	201.6	578.6	646.2
Point of Destination | Chemicals | North America
Disaggregation Of Revenue [Line Items]
Net sales	153.2	147.8	434.4	456.1
Point of Destination | Chemicals | Other
Disaggregation Of Revenue [Line Items]
Net sales	$ 69.4	$ 88.0	$ 210.9	$ 256.1