Major Customers (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Foresight Energy Lp
Concentration Risk [Line Items]
Revenues	$ 20,635	$ 18,677	$ 47,648	$ 68,556
Alpha Natural Resources [Member]
Concentration Risk [Line Items]
Revenue From Lease Assignment Fees, Nonrecurring		6,000
Revenues	$ 3,829	$ 15,429	$ 14,420	$ 33,201
Sales Revenue | Customer Concentration Risk | Foresight Energy Lp
Concentration Risk [Line Items]
Concentration risk (percent)	21.00%	16.00%	15.00%	21.00%
Sales Revenue | Customer Concentration Risk | Alpha Natural Resources [Member]
Concentration Risk [Line Items]
Concentration risk (percent)	4.00%	14.00%	5.00%	10.00%