Segment Information - Schedule of Segment Revenues for Significant Product Lines (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Net sales	$ 436.6	$ 443.8	$ 838.5	$ 820.7
Railroad and Utility Products and Services [Member]
Segment Reporting Information [Line Items]
Net sales	209.9	199.1	399.9	365.2
Railroad and Utility Products and Services [Member] | Railroad Treated Products [Member]
Segment Reporting Information [Line Items]
Net sales	125.3	115.5	237.3	204.1
Railroad and Utility Products and Services [Member] | Utility Poles [Member]
Segment Reporting Information [Line Items]
Net sales	66.0	60.0	127.8	114.5
Railroad and Utility Products and Services [Member] | Rail Joints [Member]
Segment Reporting Information [Line Items]
Net sales	6.0	8.4	12.5	16.8
Railroad and Utility Products and Services [Member] | Railroad Infrastructure Services [Member]
Segment Reporting Information [Line Items]
Net sales	8.2	10.4	14.1	19.7
Railroad and Utility Products and Services [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Net sales	4.4	4.8	8.2	10.1
Performance Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales	137.1	120.8	248.5	219.8
Performance Chemicals [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Net sales	6.8	4.0	10.6	6.6
Performance Chemicals [Member] | Wood Preservative Products [Member]
Segment Reporting Information [Line Items]
Net sales	130.3	116.8	237.9	213.2
Carbon Materials and Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales	89.6	123.9	190.1	235.7
Carbon Materials and Chemicals [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Net sales	7.5	8.5	14.0	14.3
Carbon Materials and Chemicals [Member] | Pitch and Related Products [Member]
Segment Reporting Information [Line Items]
Net sales	58.3	74.3	108.5	137.1
Carbon Materials and Chemicals [Member] | Phthalic Anhydride and Other Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales	11.6	20.0	35.1	41.0
Carbon Materials and Chemicals [Member] | Creosote and Distillates [Member]
Segment Reporting Information [Line Items]
Net sales	8.4	15.4	23.6	31.3
Carbon Materials and Chemicals [Member] | Naphthalene [Member]
Segment Reporting Information [Line Items]
Net sales	$ 3.9	$ 5.7	$ 8.9	$ 12.1