Segment and Related Information, Sales By Product Category (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 28, 2020	Jun. 30, 2019	Jun. 28, 2020	Jun. 30, 2019
Revenue from External Customer [Abstract]
Net sales	$ 357.2	$ 463.8	$ 774.7	$ 901.5
Performance Materials [Member]
Revenue from External Customer [Abstract]
Net sales	173.8	215.4	360.0	414.6
Specialty Minerals [Member]
Revenue from External Customer [Abstract]
Net sales	109.8	145.1	246.9	289.5
Refractories [Member]
Revenue from External Customer [Abstract]
Net sales	55.9	77.5	124.9	151.3
Energy Services [Member]
Revenue from External Customer [Abstract]
Net sales	17.7	25.8	42.9	46.1
Metalcasting [Member] | Performance Materials [Member]
Revenue from External Customer [Abstract]
Net sales	52.8	75.8	114.5	149.0
Household, Personal Care & Specialty Products [Member] | Performance Materials [Member]
Revenue from External Customer [Abstract]
Net sales	87.9	91.5	184.1	186.3
Environmental Products [Member] | Performance Materials [Member]
Revenue from External Customer [Abstract]
Net sales	19.9	29.0	31.4	44.9
Building Materials [Member] | Performance Materials [Member]
Revenue from External Customer [Abstract]
Net sales	13.2	19.1	30.0	34.4
Paper PCC [Member] | Specialty Minerals [Member]
Revenue from External Customer [Abstract]
Net sales	65.5	90.2	150.6	181.7
Specialty PCC [Member] | Specialty Minerals [Member]
Revenue from External Customer [Abstract]
Net sales	14.9	17.3	32.4	35.4
Ground Calcium Carbonate [Member] | Specialty Minerals [Member]
Revenue from External Customer [Abstract]
Net sales	20.6	24.8	43.2	47.1
Talc [Member] | Specialty Minerals [Member]
Revenue from External Customer [Abstract]
Net sales	8.8	12.8	20.7	25.3
Refractory Products [Member] | Refractories [Member]
Revenue from External Customer [Abstract]
Net sales	47.1	61.0	102.9	123.0
Metallurgical Products [Member] | Refractories [Member]
Revenue from External Customer [Abstract]
Net sales	$ 8.8	$ 16.5	$ 22.0	$ 28.3