Supplemental Data - Property, Plant, and Equipment, Accrued Expense, Noncurrent Liabilities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment - Net
Gross property, plant and equipment	$ 850,600,000	$ 840,200,000
Less accumulated depreciation	470,000,000.0	444,000,000.0
Net Property, Plant and Equipment	380,600,000	396,200,000
Depreciation expense	33,900,000	32,600,000	$ 28,300,000
Interest expense capitalized	200,000	200,000	$ 0.0
Accrued Expenses
Accrued salaries and employee benefits	26,200,000	23,900,000
Amounts due to customers	8,900,000	9,600,000
Accrued income taxes	500,000	5,300,000
Accrued utilities	3,000,000.0	3,900,000
Accrued interest	1,200,000	1,200,000
Other	7,200,000	11,300,000
Total	47,000,000.0	55,200,000
Noncurrent Employee Benefits
Pension benefits	57,100,000	54,000,000.0
Post-employment benefits other than pensions	36,000,000.0	38,900,000
Total	93,100,000	92,900,000
Long-term disability benefits due to Terrace Bay retirees	1,700,000	1,700,000
Other long term benefits	200,000
Land and land improvements
Property, Plant and Equipment - Net
Gross property, plant and equipment	19,400,000	19,000,000.0
Buildings
Property, Plant and Equipment - Net
Gross property, plant and equipment	165,400,000	156,000,000.0
Machinery and equipment
Property, Plant and Equipment - Net
Gross property, plant and equipment	651,000,000.0	650,300,000
Construction in progress
Property, Plant and Equipment - Net
Gross property, plant and equipment	14,800,000	14,900,000
Canada
Noncurrent Employee Benefits
Long-term disability benefits due to Terrace Bay retirees	$ 700,000	$ 800,000