Segment information - Depreciation and amortization, and capital expenditures (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information
Depreciation and amortization	$ 40.4	$ 38.8	$ 34.6
Capital expenditures	52.6	56.7	100.9
Performance Materials
Segment Reporting Information
Depreciation and amortization	19.8	16.4	11.1
Capital expenditures	36.9	39.6	65.3
Performance Chemicals
Segment Reporting Information
Depreciation and amortization	20.6	22.4	23.5
Capital expenditures	$ 15.7	$ 17.1	$ 35.6