Restructuring and Other Items, net (Details) $ in Millions	1 Months Ended		3 Months Ended			9 Months Ended
	Aug. 31, 2020 USD ($)	Jun. 30, 2020 USD ($) Mill	Sep. 27, 2020 USD ($)	Sep. 29, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 27, 2020 USD ($)	Sep. 29, 2019 USD ($)
Restructuring Charges [Abstract]
Impairment of assets			$ 1.1	$ 0.0	$ 7.5	$ 7.1	$ 7.5
Severance and other employee costs			0.3	0.0	$ 5.7	0.6	5.7
Total restructuring and other items, net			1.5	0.0		8.0	13.2
Restructuring Reserve [Roll Forward]
Restructuring liability, beginning of period						5.0
Additional provision						0.3
Cash payments						(1.2)
Restructuring liability, ending of period			4.1			4.1
Performance Materials [Member]
Restructuring Charges [Abstract]
Impairment of assets			0.0	0.0		0.0	4.2
Severance and other employee costs			0.0	0.0		0.0	2.8
Specialty Minerals [Member]
Restructuring Charges [Abstract]
Impairment of assets			1.1	0.0		7.1	1.6
Severance and other employee costs			0.3	0.0		0.6	0.9
Specialty Minerals [Member] | Paper PCC [Member]
Restructuring and Other Items, Net [Abstract]
Number of mills with lease terminations | Mill		1
Restructuring Charges [Abstract]
Impairment of assets	$ 1.1	$ 6.0
Severance and other employee costs		$ 0.3
Refractories [Member]
Restructuring Charges [Abstract]
Severance and other employee costs			0.0	0.0		0.0	0.8
Energy Services [Member]
Restructuring Charges [Abstract]
Impairment of assets			0.0	0.0		0.0	1.7
Severance and other employee costs			0.0	0.0		0.0	0.1
Corporate [Member]
Restructuring Charges [Abstract]
Severance and other employee costs			0.0	0.0		0.0	1.1
Other			$ 0.1	$ 0.0		$ 0.3	$ 0.0
Verso Papers [Member]
Restructuring and Other Items, Net [Abstract]
Number of paper mills being idled indefinitely | Mill		2