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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation Of Revenue [Line Items]
Net sales	$ 475.2	$ 455.2	$ 1,483.5	$ 1,431.4
Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	437.4	410.3	1,358.4	1,312.5
Point of Origin | Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	437.4	410.3	1,358.4	1,312.5
Point of Origin | Chemicals | Germany
Disaggregation Of Revenue [Line Items]
Net sales	238.3	211.9	706.0	704.1
Point of Origin | Chemicals | United States
Disaggregation Of Revenue [Line Items]
Net sales	227.7	234.5	762.3	640.2
Point of Origin | Chemicals | Canada
Disaggregation Of Revenue [Line Items]
Net sales	84.0	82.3	258.4	236.1
Point of Origin | Chemicals | Belgium
Disaggregation Of Revenue [Line Items]
Net sales	58.3	66.4	205.7	205.0
Point of Origin | Chemicals | Norway
Disaggregation Of Revenue [Line Items]
Net sales	46.0	49.8	145.2	159.4
Point of Origin | Chemicals | Eliminations
Disaggregation Of Revenue [Line Items]
Net sales	(216.9)	(234.6)	(719.2)	(632.3)
Point of Destination | Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	437.4	410.3	1,358.4	1,312.5
Point of Destination | Chemicals | Europe
Disaggregation Of Revenue [Line Items]
Net sales	201.6	191.8	646.2	664.2
Point of Destination | Chemicals | North America
Disaggregation Of Revenue [Line Items]
Net sales	147.8	142.0	456.1	412.6
Point of Destination | Chemicals | Other
Disaggregation Of Revenue [Line Items]
Net sales	$ 88.0	$ 76.5	$ 256.1	$ 235.7