Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Balance Beginning	$ 779.3	$ 778.7
Change in goodwill relating to [Abstract]
Acquisition of Sivomatic	35.0
Foreign exchange translation	(1.9)	0.6
Total	33.1	0.6
Balance Ending	$ 812.4	779.3
Finite-Lived Intangible Assets [Abstract]
Weighted average useful life	32 years
Gross carrying amount	$ 255.9	229.5
Accumulated amortization	41.8	33.0
Performance Materials [Member]
Goodwill [Roll Forward]
Balance Beginning	720.9	720.9
Change in goodwill relating to [Abstract]
Acquisition of Sivomatic	35.0
Foreign exchange translation	0.0	0.0
Total	35.0	0.0
Balance Ending	755.9	720.9
Specialty Minerals [Member]
Goodwill [Roll Forward]
Balance Beginning	12.7	12.1
Change in goodwill relating to [Abstract]
Acquisition of Sivomatic	0.0
Foreign exchange translation	(0.4)	0.6
Total	(0.4)	0.6
Balance Ending	12.3	12.7
Refractories [Member]
Goodwill [Roll Forward]
Balance Beginning	45.7	45.7
Change in goodwill relating to [Abstract]
Acquisition of Sivomatic	0.0
Foreign exchange translation	(1.5)	0.0
Total	(1.5)	0.0
Balance Ending	$ 44.2	45.7
Trade Names [Member]
Finite-Lived Intangible Assets [Abstract]
Weighted average useful life	35 years
Gross carrying amount	$ 204.2	199.8
Accumulated amortization	$ 26.6	20.7
Technology [Member]
Finite-Lived Intangible Assets [Abstract]
Weighted average useful life	13 years
Gross carrying amount	$ 18.8	18.8
Accumulated amortization	$ 6.4	4.8
Patents [Member]
Finite-Lived Intangible Assets [Abstract]
Weighted average useful life	19 years
Gross carrying amount	$ 6.4	6.4
Accumulated amortization	$ 5.6	5.3
Customer Relationships [Member]
Finite-Lived Intangible Assets [Abstract]
Weighted average useful life	22 years
Gross carrying amount	$ 26.5	4.5
Accumulated amortization	$ 3.2	$ 2.2