Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 27, 2020	Jun. 28, 2020	Mar. 29, 2020	Dec. 31, 2019	Sep. 29, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue from External Customer [Abstract]
Net sales	$ 431.8	$ 388.3	$ 357.2	$ 417.5	$ 440.2	$ 449.3	$ 463.8	$ 437.7	$ 1,594.8	$ 1,791.0	$ 1,807.6
Gross profit	111.3	98.4	88.9	106.8	107.7	111.2	112.0	109.7	405.4	440.6	461.4
Income from operations	54.5	48.5	27.2	57.7	47.7	53.5	45.5	62.0	187.9	208.7	255.9
Consolidated net income	32.0	29.3	14.9	39.6	29.6	39.1	27.6	40.0	115.8	136.4	173.2
Net income attributable to MTI	$ 31.1	$ 28.3	$ 14.4	$ 38.6	$ 29.0	$ 38.0	$ 26.6	$ 39.1	$ 112.4	$ 132.7	$ 169.0
Basic earnings per share attributable to MTI shareholders (in dollars per share)	$ 0.91	$ 0.83	$ 0.42	$ 1.12	$ 0.83	$ 1.09	$ 0.76	$ 1.11	$ 3.29	$ 3.79	$ 4.79
Diluted earnings per share attributable to MTI shareholders (in dollars per share)	0.91	0.83	0.42	1.12	0.83	1.08	0.75	1.11	3.29	3.78	$ 4.75
Market price range per share of common stock [Abstract]
High (in dollars per share)	64.16	53.34	55.42	58.91	58.12	55.33	63.20	61.01
Low (in dollars per share)	51.10	43.11	31.91	28.90	48.92	45.55	51.78	49.47
Close (in dollars per share)	62.12	49.23	43.20	32.48	57.63	52.77	53.51	58.79	62.12	$ 57.63
Dividends paid per common share (in dollars per share)	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.20
Performance Materials Segment [Member]
Revenue from External Customer [Abstract]
Net sales	$ 202.2	$ 190.6	$ 173.8	$ 186.2	$ 201.4	$ 207.3	$ 215.4	$ 199.2	$ 752.8	$ 823.3	$ 828.1
Specialty Minerals Segment [Member]
Revenue from External Customer [Abstract]
Net sales	138.9	125.1	109.8	137.1	141.8	143.1	145.1	144.4	510.9	574.4	589.3
Refractories Segment [Member]
Revenue from External Customer [Abstract]
Net sales	73.9	59.3	55.9	69.0	73.4	73.4	77.5	73.8	258.1	298.1	311.9
Energy Services Segment [Member]
Revenue from External Customer [Abstract]
Net sales	$ 16.8	$ 13.3	$ 17.7	$ 25.2	$ 23.6	$ 25.5	$ 25.8	$ 20.3	$ 73.0	$ 95.2	$ 78.3