Goodwill and other intangible assets, net - Intangible Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets, Net
Net	$ 4.9	$ 7.3
Brands | Performance Chemicals
Finite-Lived Intangible Assets, Net
Gross carrying amount	13.9	13.9
Accumulated amortization	11.8	11.3
Net	2.1	2.6
Customer contracts and relationships | Performance Chemicals
Finite-Lived Intangible Assets, Net
Gross carrying amount	28.2	28.2
Accumulated amortization	25.4	23.5
Net	2.8	4.7
Other Intangible Assets [Member] | Performance Chemicals
Finite-Lived Intangible Assets, Net
Gross carrying amount	42.1	42.1
Accumulated amortization	37.2	34.8
Net	$ 4.9	$ 7.3