Employee Benefits (Narrative) (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) employee	Dec. 31, 2018 USD ($)
Employer matching contribution, percent of employees' pay	100.00%
Maximum annual contribution per employee, percent	4.00%
Participants in employee health plan | employee	2,803
Health care cap per person	$ 200,000
Insured capped maximum exposure health care	17,000,000
Employee benefit costs	19,465,000	$ 15,603,000
Workers' compensation costs	3,100,000	3,000,000
Insured maximum exposure per employee	1,000,000
Aggregate insurance deductible for employees	$ 4,000,000
Percentage claims are below average of peers underground coal mining	24.00%
Operating Expense [Member]
Employee benefit costs	$ 18,900,000	$ 15,100,000