Income Taxes (Summary of the Total Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets
Tax carryforwards	$ 19,058	$ 22,035
Inventories	2,041	1,878
Accrued expenses and reserves	9,860	11,723
Other employee benefits	4,892	4,640
Other	9,347	8,933
Total deferred tax assets	45,198	49,209
Less: Valuation allowance	14,219	13,579
Deferred tax assets, net of valuation allowance	30,979	35,630
Deferred tax liabilities
Depreciation and depletion	27,299	23,029
Partnership investment - development costs	5,146	4,069
Accrued pension benefits	1,380	2,570
Total deferred tax liabilities	33,825	29,668
Net deferred liability	$ (2,846)	0
Net deferred asset		$ 5,962