Income Taxes (Summary of the Total Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets
Tax carryforwards	$ 22,035	$ 21,527
Inventories	1,878	3,389
Accrued expenses and reserves	11,723	18,750
Partnership investment - development costs	0	3,719
Other employee benefits	4,640	5,130
Other	8,933	14,737
Total deferred tax assets	49,209	67,252
Less: Valuation allowance	13,579	12,881
Deferred tax assets, net of valuation allowance	35,630	54,371
Deferred tax liabilities
Depreciation and depletion	23,029	42,512
Partnership investment - development costs	4,069	0
Accrued pension benefits	2,570	983
Total deferred tax liabilities	29,668	43,495
Net deferred asset	$ 5,962	$ 10,876