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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation Of Revenue [Line Items]
Net sales	$ 388.7	$ 455.2	$ 510.2	$ 466.0	$ 495.9	$ 496.5	$ 481.7	$ 405.3	$ 1,820.1	$ 1,879.4	$ 1,519.4
Point of Origin
Disaggregation Of Revenue [Line Items]
Net sales									1,820.1	1,879.4	1,519.4
Point of Origin | Germany
Disaggregation Of Revenue [Line Items]
Net sales									886.1	918.6	699.8
Point of Origin | United States
Disaggregation Of Revenue [Line Items]
Net sales									997.6	992.3	819.3
Point of Origin | Canada
Disaggregation Of Revenue [Line Items]
Net sales									307.2	309.2	257.7
Point of Origin | Belgium
Disaggregation Of Revenue [Line Items]
Net sales									272.2	279.9	187.4
Point of Origin | Norway
Disaggregation Of Revenue [Line Items]
Net sales									209.6	216.4	164.8
Point of Origin | Eliminations
Disaggregation Of Revenue [Line Items]
Net sales									(852.6)	(837.0)	(609.6)
Point of Destination
Disaggregation Of Revenue [Line Items]
Net sales									1,820.1	1,879.4	1,519.4
Point of Destination | Europe
Disaggregation Of Revenue [Line Items]
Net sales									817.6	899.2	698.2
Point of Destination | North America
Disaggregation Of Revenue [Line Items]
Net sales									698.7	668.3	566.8
Chemicals
Disaggregation Of Revenue [Line Items]
Net sales									1,661.9	1,729.0	1,364.3
Chemicals | Point of Origin
Disaggregation Of Revenue [Line Items]
Net sales									1,661.9	1,729.0	1,364.3
Chemicals | Point of Origin | Germany
Disaggregation Of Revenue [Line Items]
Net sales									886.1	918.6	699.8
Chemicals | Point of Origin | United States
Disaggregation Of Revenue [Line Items]
Net sales									839.4	841.8	664.2
Chemicals | Point of Origin | Canada
Disaggregation Of Revenue [Line Items]
Net sales									307.2	309.2	257.7
Chemicals | Point of Origin | Belgium
Disaggregation Of Revenue [Line Items]
Net sales									272.2	279.9	187.4
Chemicals | Point of Origin | Norway
Disaggregation Of Revenue [Line Items]
Net sales									209.6	216.4	164.8
Chemicals | Point of Origin | Eliminations
Disaggregation Of Revenue [Line Items]
Net sales									(852.6)	(836.9)	(609.6)
Chemicals | Point of Destination
Disaggregation Of Revenue [Line Items]
Net sales									1,661.9	1,729.0	1,364.3
Chemicals | Point of Destination | Europe
Disaggregation Of Revenue [Line Items]
Net sales									817.2	898.8	697.6
Chemicals | Point of Destination | North America
Disaggregation Of Revenue [Line Items]
Net sales									542.0	519.4	413.2
Chemicals | Point of Destination | Other
Disaggregation Of Revenue [Line Items]
Net sales									$ 302.7	$ 310.8	$ 253.5