Derivative Instruments and Fair Value Measurements - Derivatives by Hedging Relationship and Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Derivatives:
Loss on deconsolidation of subsidiary (Note 3)	$ 1,635	$ 0	$ 1,635	$ 0
Fair Value, Measurements, Recurring
Derivatives:
Current assets	782		782		$ 112
Current assets, counterparty offsets	(400)		(400)		(266)
Current liabilities	(34)		(34)		(64)
Current liabilities, counterparty offsets	400		400		266
Net value	748		748		48
Net value, counterparty offsets	0		0		0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1)
Derivatives:
Current assets	0		0		0
Current liabilities	0		0		0
Net value	0		0		0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Derivatives:
Current assets	1,182		1,182		378
Current liabilities	(434)		(434)		(330)
Net value	748		748		48
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member]
Derivatives:
Current assets	0		0		0
Current liabilities	0		0		0
Net value	0		0		$ 0
Fair Value, Measurements, Nonrecurring
Derivatives:
Investment in AREC	5,070		5,070
Loss on deconsolidation of subsidiary (Note 3)			1,635
Fair Value, Measurements, Nonrecurring | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1)
Derivatives:
Investment in AREC	0		0
Fair Value, Measurements, Nonrecurring | Significant Unobservable Inputs (Level 3)
Derivatives:
Investment in AREC	5,070		5,070
Fair Value, Measurements, Nonrecurring | Fair Value, Inputs, Level 3 [Member]
Derivatives:
Investment in AREC	0		0
Commodity Contract | Revenues - Marketing | Not Designated as Hedging Instrument
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Revenues – Marketing	$ 280	$ 282	$ 700	$ 124