Restructuring and Other Items, net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Cost and Reserve [Line Items]
Annualized savings from reduction of workforce	$ 6.0	$ 11.5
Restructuring Charges [Abstract]
Impairment of assets charges	5.3	18.5	$ 34.2
Severance and other employee costs	10.8	12.7	11.0
Total restructuring and other items, net	15.0	28.3	45.2
Restructuring Reserve [Roll Forward]
Restructuring liability, beginning of period	3.6
Additional provisions	10.8
Cash payments	(6.3)
Restructuring liability, ending of period	8.1	3.6
Specialty Minerals [Member]
Restructuring Charges [Abstract]
Impairment of assets charges	5.3	0.0	0.0
Severance and other employee costs	5.0	0.0	0.0
Performance Materials [Member]
Restructuring Charges [Abstract]
Impairment of assets charges	0.0	0.0	0.0
Severance and other employee costs	0.0	0.0	0.0
Refractories [Member]
Restructuring Cost and Reserve [Line Items]
Gain on previously impaired assets		2.9
Restructuring Charges [Abstract]
Severance and other employee costs	0.0	0.0	2.0
Other	0.0	(2.0)	0.0
Energy Services [Member]
Restructuring Charges [Abstract]
Impairment of assets charges	0.0	18.5	33.0
Severance and other employee costs	1.7	12.7	9.0
Other	(1.1)	(0.9)	0.0
Corporate [Member]
Restructuring Charges [Abstract]
Impairment of assets charges	0.0	0.0	1.2
Severance and other employee costs	$ 4.1	$ 0.0	$ 0.0