Employee Benefits (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) employee	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Employer matching contribution, percent of employees' pay	100.00%
Maximum annual contribution per employee, percent	4.00%
Participants in employee health plan | employee	2,289
Health care cap per person	$ 200,000
Insured capped maximum exposure health care	14,400,000
Employee benefit costs	15,880,000	$ 14,718,000	$ 16,112,000
Workers' compensation costs	2,800,000	2,300,000	4,600,000
Insured maximum exposure per employee	1,000,000
Aggragate insurance deductable for employees	$ 4,000,000
Percentage claims are below average of peers underground coal mining	27.00%
Operating Expense [Member]
Employee benefit costs	$ 15,500,000	13,800,000	15,200,000
Workers' compensation costs	$ 2,500,000	$ 2,300,000	$ 4,600,000