Revenue - Disaggregation of Sales - Schedule of Disaggregates of Net Sales of our Chemicals Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation Of Revenue [Line Items]
Net sales	$ 479.6	$ 466.0
Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	436.5	430.4
Point of Origin | Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	436.5	430.4
Point of Origin | Chemicals | Germany
Disaggregation Of Revenue [Line Items]
Net sales	219.2	234.5
Point of Origin | Chemicals | United States
Disaggregation Of Revenue [Line Items]
Net sales	245.3	196.8
Point of Origin | Chemicals | Canada
Disaggregation Of Revenue [Line Items]
Net sales	78.5	71.6
Point of Origin | Chemicals | Belgium
Disaggregation Of Revenue [Line Items]
Net sales	69.7	69.7
Point of Origin | Chemicals | Norway
Disaggregation Of Revenue [Line Items]
Net sales	51.5	53.1
Point of Origin | Chemicals | Eliminations
Disaggregation Of Revenue [Line Items]
Net sales	(227.7)	(195.3)
Point of Destination | Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	436.5	430.4
Point of Destination | Chemicals | Europe
Disaggregation Of Revenue [Line Items]
Net sales	215.2	233.9
Point of Destination | Chemicals | North America
Disaggregation Of Revenue [Line Items]
Net sales	146.8	127.0
Point of Destination | Chemicals | Other
Disaggregation Of Revenue [Line Items]
Net sales	$ 74.5	$ 69.5