Property, Plant, and Equipment (Tables)

12 Months Ended

Dec. 31, 2017

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)	2017			2016
Land	$	4,874		$	5,548
Buildings	137,196			135,729
Machinery and equipment	626,186			609,894
Software	40,575			39,550
Construction in progress	29,963			19,111
Allowances for depreciation	(615,134		)	(593,531		)
Subtotal	223,660			216,301
Capital leases	10,912			10,913
Allowances for depreciation	(2,741		)	(2,492		)
Subtotal	8,171			8,421
Mineral resources	4,979			4,979
Mine development	37,103			35,543
Allowances for amortization and depletion	(18,335		)	(12,613		)
Subtotal	23,747			27,909
Property, plant, and equipment — net	$	255,578		$	252,631