Supplemental Data - Property, Plant, and Equipment, Accrued Expense, Noncurrent Liabilities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment - Net
Gross property, plant and equipment	$ 840,200,000	$ 850,500,000
Less accumulated depreciation	444,000,000	425,300,000
Net Property, Plant and Equipment	396,200,000	425,200,000
Depreciation expense	32,600,000	28,300,000	$ 27,100,000
Interest expense capitalized	200,000	0	$ 800,000
Accrued Expenses
Accrued salaries and employee benefits	23,900,000	29,600,000
Amounts due to customers	9,600,000	7,200,000
Accrued utilities	1,200,000	1,300,000
Accrued interest	5,300,000	4,200,000
Accrued utilities	3,900,000	3,700,000
Other	11,300,000	11,500,000
Total	55,200,000	57,500,000
Noncurrent Employee Benefits
Pension benefits	54,000,000	59,800,000
Post-employment benefits other than pensions	38,900,000	40,500,000
Total	92,900,000	100,300,000
Long-term disability benefits due to Terrace Bay retirees	1,700,000	1,800,000
Other long term liabilities	200,000
Land and land improvements
Property, Plant and Equipment - Net
Gross property, plant and equipment	19,000,000	20,200,000
Buildings
Property, Plant and Equipment - Net
Gross property, plant and equipment	156,000,000	157,700,000
Machinery and equipment
Property, Plant and Equipment - Net
Gross property, plant and equipment	650,300,000	650,800,000
Construction in progress
Property, Plant and Equipment - Net
Gross property, plant and equipment	$ 14,900,000	$ 21,800,000