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Income Taxes (Summary of the Total Deferred Tax Assets and Liabilities) (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Deferred tax assets    
Lease liabilities $ 30,875  
Tax carryforwards 16,305 $ 19,058
Inventories 1,704 2,041
Accrued liabilities 10,020 9,860
Employee benefits 4,853 4,892
Other 9,005 9,347
Total deferred tax assets 72,762 45,198
Less: Valuation allowance 12,296 14,219
Deferred tax assets, net of valuation allowance 60,466 30,979
Deferred Tax Liabilities, Lease Right-Of-Use Assets 30,875  
Deferred tax liabilities    
Depreciation and depletion 28,061 27,299
Partnership investment - development costs 9,949 5,146
Accrued pension benefits 3,919 1,380
Total deferred tax liabilities 72,804 33,825
Net deferred liability $ (12,338) $ (2,846)