Disaggregation of Sales - Schedule of Disaggregates of Net Sales of our Chemicals Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation Of Revenue [Line Items]
Net sales	$ 516.8	$ 458.6	$ 415.0	$ 459.3	$ 414.1	$ 475.2	$ 528.6	$ 479.6	$ 1,849.7	$ 1,897.5	$ 1,820.1
Point of Origin
Disaggregation Of Revenue [Line Items]
Net sales									1,849.7	1,897.5	1,820.1
Point of Origin | Germany
Disaggregation Of Revenue [Line Items]
Net sales									836.0	883.6	886.1
Point of Origin | U.S.
Disaggregation Of Revenue [Line Items]
Net sales									1,189.8	1,164.8	997.6
Point of Origin | Canada
Disaggregation Of Revenue [Line Items]
Net sales									319.5	328.7	307.2
Point of Origin | Belgium
Disaggregation Of Revenue [Line Items]
Net sales									249.5	270.7	272.2
Point of Origin | Norway
Disaggregation Of Revenue [Line Items]
Net sales									211.8	192.2	209.6
Point of Origin | Eliminations
Disaggregation Of Revenue [Line Items]
Net sales									(956.9)	(942.5)	(852.6)
Point of Destination
Disaggregation Of Revenue [Line Items]
Net sales									1,849.7	1,897.5	1,820.1
Point of Destination | Europe
Disaggregation Of Revenue [Line Items]
Net sales									783.8	824.2	817.6
Point of Destination | North America
Disaggregation Of Revenue [Line Items]
Net sales									778.2	740.1	698.7
Chemicals
Disaggregation Of Revenue [Line Items]
Net sales									1,638.8	1,731.2	1,661.9
Chemicals | Point of Origin
Disaggregation Of Revenue [Line Items]
Net sales									1,638.8	1,731.2	1,661.9
Chemicals | Point of Origin | Germany
Disaggregation Of Revenue [Line Items]
Net sales									836.0	883.6	886.1
Chemicals | Point of Origin | U.S.
Disaggregation Of Revenue [Line Items]
Net sales									978.8	998.5	839.4
Chemicals | Point of Origin | Canada
Disaggregation Of Revenue [Line Items]
Net sales									319.5	328.7	307.2
Chemicals | Point of Origin | Belgium
Disaggregation Of Revenue [Line Items]
Net sales									249.5	270.7	272.2
Chemicals | Point of Origin | Norway
Disaggregation Of Revenue [Line Items]
Net sales									211.8	192.2	209.6
Chemicals | Point of Origin | Eliminations
Disaggregation Of Revenue [Line Items]
Net sales									(956.8)	(942.5)	(852.6)
Chemicals | Point of Destination
Disaggregation Of Revenue [Line Items]
Net sales									1,638.8	1,731.2	1,661.9
Chemicals | Point of Destination | Europe
Disaggregation Of Revenue [Line Items]
Net sales									783.2	823.5	817.2
Chemicals | Point of Destination | North America
Disaggregation Of Revenue [Line Items]
Net sales									569.3	575.6	542.0
Chemicals | Point of Destination | Other
Disaggregation Of Revenue [Line Items]
Net sales									$ 286.3	$ 332.1	$ 302.7