Segment Information - Summary of Segment Revenues for Significant Product Lines (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting [Line Items]
Net sales	$ 520.1	$ 504.8	$ 975.4	$ 961.3
Operating Segments
Segment Reporting [Line Items]
Net sales	520.1	504.8	975.4	961.3
Railroad and Utility Products and Services \| Operating Segments
Segment Reporting [Line Items]
Net sales	245.9	250.4	465.9	485.4
Railroad and Utility Products and Services \| Railroad treated products \| Operating Segments
Segment Reporting [Line Items]
Net sales	147.3	150.9	281.5	297.2
Railroad and Utility Products and Services \| Utility poles \| Operating Segments
Segment Reporting [Line Items]
Net sales	86.7	75.8	161.5	143.7
Railroad and Utility Products and Services \| Railroad infrastructure products and services \| Operating Segments
Segment Reporting [Line Items]
Net sales	11.9	23.7	22.9	44.5
Performance Chemicals \| Wood preservative products and other \| Operating Segments
Segment Reporting [Line Items]
Net sales	168.2	150.8	310.3	271.7
Carbon Materials and Chemicals \| Operating Segments
Segment Reporting [Line Items]
Net sales	106.0	103.6	199.2	204.2
Carbon Materials and Chemicals \| Pitch and related products \| Operating Segments
Segment Reporting [Line Items]
Net sales	75.0	74.4	145.2	139.6
Carbon Materials and Chemicals \| Carbon black feedstock and distillates \| Operating Segments
Segment Reporting [Line Items]
Net sales	22.9	14.4	36.7	28.6
Carbon Materials and Chemicals \| Naphthalene, phthalic anhydride, and other chemicals \| Operating Segments
Segment Reporting [Line Items]
Net sales	$ 8.1	$ 14.8	$ 17.3	$ 36.0