Fair Value Disclosure (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain on equity securities	$ 1,226	$ 1,545
Long-term debt fair value	45,200	24,300
Long-term Debt	44,997	24,300
Mineral Management
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived asset write off	7,300
Coal Mining customer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived asset write off	1,100
Level 1 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain on equity securities	100
Level 1 | Fair Value, Measurements, Recurring | Reported Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in equity securities	2,000
Level 1 | Fair Value, Measurements, Recurring | Bellaire
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain on equity securities	$ 1,200	$ 1,500