Segment information - Customer Sales (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue, Major Customer [Line Items]
Net sales	$ 229.5	$ 264.1	$ 260.3	$ 218.5	$ 210.9	$ 252.4	$ 245.4	$ 199.6	$ 972.4	$ 908.3	$ 958.3
Performance Materials
Revenue, Major Customer [Line Items]
Net sales									349.3	301.0	256.4
Performance Materials | Automotive Technologies product line
Revenue, Major Customer [Line Items]
Net sales									312.5	263.5	222.5
Performance Materials | Process Purification product line
Revenue, Major Customer [Line Items]
Net sales									36.8	37.5	33.9
Performance Chemicals
Revenue, Major Customer [Line Items]
Net sales									623.1	607.3	701.9
Performance Chemicals | Pavement Technologies product line
Revenue, Major Customer [Line Items]
Net sales									163.0	148.8	147.5
Performance Chemicals | Oilfield Technologies product line
Revenue, Major Customer [Line Items]
Net sales									77.8	58.5	78.0
Performance Chemicals | Industrial Specialties product line
Revenue, Major Customer [Line Items]
Net sales									$ 382.3	$ 400.0	$ 476.4