Income Taxes (Summary of the Total Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets
Lease liabilities	$ 27,800	$ 30,875
Tax carryforwards	17,756	16,305
Inventories	3,742	1,704
Accrued liabilities	10,160	10,020
Employee benefits	2,747	4,853
Land valuation adjustment	5,536	4,304
Other	5,401	4,701
Total deferred tax assets	73,142	72,762
Less: Valuation allowance	11,549	12,296
Deferred tax assets, net of valuation allowance	61,593	60,466
Deferred tax liabilities
Lease right-of-use assets	27,800	30,875
Depreciation and depletion	31,972	28,061
Partnership investment - development costs	11,686	9,949
Accrued pension benefits	7,685	3,919
Total deferred tax liabilities	79,143	72,804
Net deferred liability	$ (17,550)	$ (12,338)