Revenue Recognition - Schedule of Disaggregates of Net Sales of our Chemicals Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation Of Revenue [Line Items]
Net sales	$ 510.2	$ 481.8	$ 976.2	$ 887.1
Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	471.8	441.4	902.2	811.2
Point of Origin | Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	471.8	441.4	902.2	811.2
Point of Origin | Chemicals | Germany
Disaggregation Of Revenue [Line Items]
Net sales	257.7	231.9	492.2	415.5
Point of Origin | Chemicals | United States
Disaggregation Of Revenue [Line Items]
Net sales	208.9	208.6	405.7	414.3
Point of Origin | Chemicals | Canada
Disaggregation Of Revenue [Line Items]
Net sales	82.2	79.9	153.8	157.8
Point of Origin | Chemicals | Belgium
Disaggregation Of Revenue [Line Items]
Net sales	68.9	72.0	138.6	130.1
Point of Origin | Chemicals | Norway
Disaggregation Of Revenue [Line Items]
Net sales	56.5	51.3	109.6	98.6
Point of Origin | Chemicals | Eliminations
Disaggregation Of Revenue [Line Items]
Net sales	(202.4)	(202.3)	(397.7)	(405.1)
Point of Destination | Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	471.8	441.4	902.2	811.2
Point of Destination | Chemicals | Europe
Disaggregation Of Revenue [Line Items]
Net sales	238.5	226.6	472.4	406.3
Point of Destination | Chemicals | North America
Disaggregation Of Revenue [Line Items]
Net sales	143.6	138.1	270.6	262.1
Point of Destination | Chemicals | Other
Disaggregation Of Revenue [Line Items]
Net sales	$ 89.7	$ 76.7	$ 159.2	$ 142.8