Goodwill and Other Intangible Assets, Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Balance Beginning	$ 807.4	$ 812.4
Change in goodwill relating to [Abstract]
Foreign exchange translation	1.1	(5.0)
Total changes	1.1	(5.0)
Balance Ending	808.5	807.4
Performance Materials [Member]
Goodwill [Roll Forward]
Balance Beginning	751.2	755.9
Change in goodwill relating to [Abstract]
Foreign exchange translation	1.2	(4.7)
Total changes	1.2	(4.7)
Balance Ending	752.4	751.2
Specialty Minerals [Member]
Goodwill [Roll Forward]
Balance Beginning	12.5	12.3
Change in goodwill relating to [Abstract]
Foreign exchange translation	0.2	0.2
Total changes	0.2	0.2
Balance Ending	12.7	12.5
Refractories [Member]
Goodwill [Roll Forward]
Balance Beginning	43.7	44.2
Change in goodwill relating to [Abstract]
Foreign exchange translation	(0.3)	(0.5)
Total changes	(0.3)	(0.5)
Balance Ending	$ 43.4	$ 43.7