Property, Plant, and Equipment (Tables)

12 Months Ended

Dec. 31, 2019

Property, Plant and Equipment [Abstract]

Summary of Property, Plant and Equipment

Property, plant, and equipment on the Consolidated Balance Sheets is summarized as follows:

	December 31,
(Thousands)	2019			2018
Land	$	4,874		$	4,874
Buildings	150,323			149,701
Machinery and equipment	639,310			631,421
Software	44,652			42,678
Construction in progress	16,699			14,468
Allowances for depreciation	(669,250		)	(642,365		)
Subtotal	186,608			200,777
Finance leases	26,069			22,150
Allowances for depreciation	(3,569		)	(2,412		)
Subtotal	22,500			19,738
Mineral resources	4,980			4,980
Mine development	30,058			27,979
Allowances for amortization and depletion	(11,870		)	(2,456		)
Subtotal	23,168			30,503
Property, plant, and equipment — net	$	232,276		$	251,018