Restructuring and Other Items, net (Details) $ in Millions	3 Months Ended			12 Months Ended
	Sep. 27, 2020 USD ($)	Jun. 28, 2020 USD ($) Mill	Mar. 31, 2019 USD ($) Facility	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Restructuring Charges [Abstract]
Asset write-down charges				$ 7.1	$ 7.5	$ 0.7
Severance and other employee costs				0.3	5.7	1.8
Total restructuring and other items, net				7.6	13.2	2.5
Restructuring Reserve [Roll Forward]
Restructuring liability, beginning of period			$ 2.5	5.0	2.5
Additional provisions				0.3	5.7
Cash payments				(1.6)	(3.2)
Other				(0.1)	0.0
Restructuring liability, ending of period				3.6	5.0	2.5
Performance Materials [Member]
Restructuring Charges [Abstract]
Asset write-down charges				0.0	4.2	0.0
Severance and other employee costs				0.0	2.8	0.0
Specialty Minerals [Member]
Restructuring Charges [Abstract]
Asset write-down charges				7.1	1.6	0.7
Severance and other employee costs				0.3	0.9	0.0
Other				0.2	0.0	0.0
Specialty Minerals [Member] | Paper PCC [Member]
Restructuring and Other Items, Net [Abstract]
Number of mills with lease terminations | Mill		1
Number of facilities shut down | Facility			1
Restructuring Charges [Abstract]
Asset write-down charges	$ 1.1	$ 6.0
Severance and other employee costs		$ 0.3
Refractories [Member]
Restructuring Charges [Abstract]
Severance and other employee costs				0.0	0.8	0.0
Energy Services [Member]
Restructuring Charges [Abstract]
Asset write-down charges				0.0	1.7	0.0
Severance and other employee costs				0.0	0.1	1.8
Corporate [Member]
Restructuring Charges [Abstract]
Severance and other employee costs				$ 0.0	$ 1.1	$ 0.0
Verso Papers [Member]
Restructuring and Other Items, Net [Abstract]
Number of paper mills being idled indefinitely | Mill		2