Major Customers - Summary of Major Customers (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Revenue, Major Customer [Line Items]
Receivable balance	$ 15,333		$ 15,333		$ 20,386
Sales	54,923	$ 54,688	109,723	$ 106,232
Javelin Global [Member]
Revenue, Major Customer [Line Items]
Receivable balance	3,727		3,727		2,470
Sales			16,554
Dominion Energy [Member]
Revenue, Major Customer [Line Items]
Receivable balance	1,958		1,958		1,232
Sales			14,013	11,123
Integrity Coal [Member]
Revenue, Major Customer [Line Items]
Receivable balance					2,238
Sales			11,364	12,268
Big Rivers Electric Corporation [Member]
Revenue, Major Customer [Line Items]
Receivable balance	932		932
Sales			10,817
LG&E [Member]
Revenue, Major Customer [Line Items]
Receivable balance	$ 234		234		$ 1,483
Sales			$ 6,559	21,162
Big Rivers Electric Corporation [Member]
Revenue, Major Customer [Line Items]
Sales				$ 13,234