Restructuring and Other Items, net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring, settlement and impairment provisions [Abstract]
Annualized savings from reduction of workforce		$ 6.0
Restructuring Charges [Abstract]
Impairment of assets charges	$ 0.7	5.3	$ 18.5
Severance and other employee costs	1.8	10.8	12.7
Total restructuring and other items, net	2.5	15.0	28.3
Restructuring Reserve [Roll Forward]
Restructuring liability, beginning of period	8.1
Additional provisions	1.7
Cash payments	(6.6)
Other	(0.7)
Restructuring liability, ending of period	2.5	8.1
Specialty Minerals [Member]
Restructuring Charges [Abstract]
Impairment of assets charges	0.7	5.3	0.0
Severance and other employee costs	0.0	5.0	0.0
Refractories [Member]
Restructuring, settlement and impairment provisions [Abstract]
Gain on previously impaired assets			2.9
Restructuring Charges [Abstract]
Other	0.0	0.0	(2.0)
Energy Services [Member]
Restructuring Charges [Abstract]
Impairment of assets charges	0.0	0.0	18.5
Severance and other employee costs	1.8	1.7	12.7
Other	0.0	(1.1)	(0.9)
Corporate [Member]
Restructuring Charges [Abstract]
Impairment of assets charges	0.0	0.0	0.0
Severance and other employee costs	$ 0.0	$ 4.1	$ 0.0