Income Taxes (Summary of the Total Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets
Tax carryforwards	$ 24,138	$ 12,812
Inventories	3,810	4,680
Accrued expenses and reserves	27,777	30,640
Other employee benefits	10,451	14,253
Partnership investment - development costs	3,818	21,766
Other	16,091	16,170
Total deferred tax assets	86,085	100,321
Less: Valuation allowance	14,495	11,723
Deferred tax assets, net of valuation allowance	71,590	88,598
Deferred tax liabilities
Depreciation and depletion	41,055	42,679
Accrued pension benefits	1,813	3,610
Unremitted foreign earnings	342	296
Total deferred tax liabilities	43,210	46,585
Net deferred asset	$ 28,380	$ 42,013