Note 5 - Fair Value of Financial Instruments (Details Textual) $ in Millions					1 Months Ended	3 Months Ended		4 Months Ended	5 Months Ended	6 Months Ended
	Mar. 24, 2016 Boe $ / item	Feb. 24, 2016 BTU $ / item	Mar. 14, 2015 BTU $ / BTU	Mar. 06, 2014 BTU $ / BTU	Mar. 31, 2016 Boe $ / item	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Feb. 28, 2016 $ / item
NGI Swap Contract [Member]
Derivative, Nonmonetary Notional Amount, Energy Measure | BTU			2.4	2,200,000
Underlying, Derivative Energy Measure | $ / BTU			3	4.95
Henry Hub Natural Gas Future ("NG") Contracts [Member] | Call Option [Member]
Proceeds from Derivative Instrument, Investing Activities | $									$ 1.9
Henry Hub Natural Gas Future ("NG") Contracts [Member]
Derivative, Nonmonetary Notional Amount, Energy Measure | BTU		4,100,000
Derivative, Price Risk Option Strike Price		2
Brent Oil Future Contracts [Member] | Call Option [Member]
Proceeds from Derivative Instrument, Investing Activities | $								$ 1.1
Brent Oil Future Contracts [Member] | Minimum [Member]
Derivative, Price Risk Option Strike Price	32.8
Brent Oil Future Contracts [Member] | Maximum [Member]
Derivative, Price Risk Option Strike Price	35.5
Brent Oil Future Contracts [Member]
Derivative, Nonmonetary Notional Amount, Energy Measure | Boe	185,000
Rolled Two Existing Options [Member] | Minimum [Member]
Derivative, Price Risk Option Strike Price					41
Rolled Two Existing Options [Member] | Maximum [Member]
Derivative, Price Risk Option Strike Price					42.5
Rolled Two Existing Options [Member]
Derivative, Nonmonetary Notional Amount, Energy Measure | Boe					31,800
Before Rolling Two Existing Options [Member] | Minimum [Member]
Derivative, Price Risk Option Strike Price												32.8
Before Rolling Two Existing Options [Member] | Maximum [Member]
Derivative, Price Risk Option Strike Price												33.5
Short Risk Reversal Transactions, Rolling Existing Call Option 1 [Member] | Minimum [Member]
Derivative, Price Risk Option Strike Price					28.5
Short Risk Reversal Transactions, Rolling Existing Call Option 1 [Member] | Maximum [Member]
Derivative, Price Risk Option Strike Price					37.5
Short Risk Reversal Transactions, Rolling Existing Call Option 1 [Member]
Derivative, Nonmonetary Notional Amount, Energy Measure | Boe					16,500
Short Risk Reversal Transactions, Rolling Existing Call Option 2 [Member] | Minimum [Member]
Derivative, Price Risk Option Strike Price					28
Short Risk Reversal Transactions, Rolling Existing Call Option 2 [Member] | Maximum [Member]
Derivative, Price Risk Option Strike Price					38.5
Short Risk Reversal Transactions, Rolling Existing Call Option 2 [Member]
Derivative, Nonmonetary Notional Amount, Energy Measure | Boe					17,000
Foreign Currency Gain (Loss) [Member]
Gain (Loss) on Price Risk Derivative Instruments Not Designated as Hedging Instruments | $							$ (1.0)				$ (2.2)
Electricity Revenues [Member]
Gain (Loss) on Price Risk Derivative Instruments Not Designated as Hedging Instruments | $							$ 0.1				$ 0.4
Derivative, Number of Options Rolled					2
Gain (Loss) on Price Risk Derivative Instruments Not Designated as Hedging Instruments | $						$ (3.9)				$ (2.2)