Income Taxes (Summary of the Total Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets
Lease liabilities	$ 24,500	$ 27,800
Tax carryforwards	13,837	17,756
Inventories	4,522	3,742
Accrued liabilities	9,243	10,160
Employee benefits	3,496	2,747
Land valuation adjustment	5,988	5,536
Other	6,527	5,401
Total deferred tax assets	68,113	73,142
Less: Valuation allowance	11,695	11,549
Deferred tax assets, net of valuation allowance	56,418	61,593
Deferred tax liabilities
Lease right-of-use assets	24,500	27,800
Depreciation and depletion	25,851	31,972
Partnership investment - development costs	9,840	11,686
Accrued pension benefits	10,941	7,685
Total deferred tax liabilities	71,132	79,143
Net deferred liability	$ (14,714)	$ (17,550)