Derivatives Financial Instruments (Tables)

9 Months Ended

Sep. 30, 2017

Derivative [Line Items]

Schedule of derivative financial instruments

Settlement Period	Notional Amount	Weighted-Average per Barrel
2017 swap positions	121	$	55.00

Schedule of offsetting and balance sheet presentation

	September 30, 2017
	Gross Amounts Recognized				Gross Amounts Offset in the Consolidated Balance Sheet				Net Amounts Presented in the Consolidated Balance Sheet
	Assets		Liabilities		Assets		Liabilities		Assets		Liabilities
WTI derivative financial instruments	$	—	$	(382)	$	—	$	—	$	—	$	(382)

	December 31, 2016
	Gross Amounts Recognized				Gross Amounts Offset in the Consolidated Balance Sheet				Net Amounts Presented in the Consolidated Balance Sheet
	Assets		Liabilities		Assets		Liabilities		Assets		Liabilities
WTI derivative financial instruments	$	752	$	—	$	—	$	—	$	752	$	—

Schedule of derivative mark-to-market (gains) and losses recognized in the consolidated statement of operations and comprehensive income

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2017		2016		2017		2016
International coal forward contracts	$	—	$	(4)	$	—	$	(54)
Domestic coal futures contracts		—		(58)		—		(55)
WTI derivative financial instruments		(838)		1,130		3,102		(5,148)
Net derivative financial instruments loss (gain)	$	(838)	$	1,068	$	3,102	$	(5,257)