Revenue - Disaggregation of Sales - Schedule of Disaggregates of Net Sales of our Chemicals Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation Of Revenue [Line Items]
Net sales	$ 415.0	$ 528.6	$ 874.3	$ 1,008.2
Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	386.0	484.5	807.0	921.0
Point of Origin | Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	386.0	484.5	807.0	921.0
Point of Origin | Chemicals | Germany
Disaggregation Of Revenue [Line Items]
Net sales	183.1	248.5	412.2	467.7
Point of Origin | Chemicals | United States
Disaggregation Of Revenue [Line Items]
Net sales	226.6	289.3	463.6	534.6
Point of Origin | Chemicals | Canada
Disaggregation Of Revenue [Line Items]
Net sales	85.6	95.9	153.3	174.4
Point of Origin | Chemicals | Belgium
Disaggregation Of Revenue [Line Items]
Net sales	50.9	77.7	117.0	147.4
Point of Origin | Chemicals | Norway
Disaggregation Of Revenue [Line Items]
Net sales	62.6	47.7	116.9	99.2
Point of Origin | Chemicals | Eliminations
Disaggregation Of Revenue [Line Items]
Net sales	(222.8)	(274.6)	(456.0)	(502.3)
Point of Destination | Chemicals
Disaggregation Of Revenue [Line Items]
Net sales	386.0	484.5	807.0	921.0
Point of Destination | Chemicals | Europe
Disaggregation Of Revenue [Line Items]
Net sales	169.8	229.4	384.3	444.6
Point of Destination | Chemicals | North America
Disaggregation Of Revenue [Line Items]
Net sales	148.9	161.5	281.2	308.3
Point of Destination | Chemicals | Other
Disaggregation Of Revenue [Line Items]
Net sales	$ 67.3	$ 93.6	$ 141.5	$ 168.1