Property, Plant, and Equipment Property, Plant, and Equipment Textuals (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Reimbursement Revenue	$ 63,500
Unearned Income	(4,500)	$ (4,600)
Depreciation and depletion expense	42,751	45,651	$ 38,471
Net book value of capitalized software	8,300	9,800
Software amortization	2,400	2,400	2,300
Property, Plant and Equipment, Other Types
Property, Plant and Equipment [Line Items]
Depreciation and depletion expense	$ 38,100	$ 41,200	$ 32,800