Segment information - Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information
Assets	$ 929.6	$ 832.8	$ 778.7
Operating Segments
Segment Reporting Information
Assets	918.7	828.1	775.7
Operating Segments | Performance Materials
Segment Reporting Information
Assets	438.9	377.2	355.2
Operating Segments | Performance Chemicals
Segment Reporting Information
Assets	479.8	450.9	420.5
Corporate and other
Segment Reporting Information
Assets	$ 10.9	$ 4.7	$ 3.0