Note 4 - Unconsolidated Investments (Details Textual)			3 Months Ended		6 Months Ended
	Jun. 04, 2014 USD ($)	May 23, 2014 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) MWh	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	May 16, 2014 USD ($)
Sarulla [Member] | Lenders Consortium [Member] | Subject to Fixed Interest Rate [Member]
Proceeds from Issuance of Senior Long-term Debt		$ 100,000,000
Sarulla [Member] | Lenders Consortium [Member] | Subject to LIBOR based Interest Rate [Member]
Senior Notes								$ 1,070,000,000
Sarulla [Member] | Lenders Consortium [Member] | Subject to Fixed LIBOR Interest Rate [Member] | Interest Rate Swap [Member]
Senior Notes							$ 960,000,000
Sarulla [Member] | Lenders Consortium [Member] | Interest Rate Swap [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate	3.4565%
Sarulla [Member] | Lenders Consortium [Member] | Interest Rate Swap [Member]
Proceeds from Issuance of Senior Long-term Debt	$ 50,000,000
Sarulla [Member] | Lenders Consortium [Member]
Senior Notes								$ 1,170,000,000
Sarulla [Member] | Interest Rate Swap [Member] | Sarulla Project Company [Member]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax			$ (15,600,000)		$ (40,500,000)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax			(8,000,000)		(20,900,000)
Sarulla [Member] | Interest Rate Swap [Member]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax			(2,000,000)		(5,200,000)
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax			$ 12,300,000		$ 12,300,000
Sarulla [Member]
Jointly Owned Utility Plant, Proportionate Ownership Share			12.75%		12.75%
Expected Power Generating Capacity | MWh					330
Power Plant Usage Agreement Term					30 years
Payments to Acquire Projects					$ 0
Number Of Phases Of Construction					3
Power Utilization | MWh					110
Percentage of Required Production Capacity			70.00%		70.00%
Percentage of Required Injection Capacity			15.00%		15.00%
Number of Commercial Lenders in Funding Consortium								6
Supply Commitment, Remaining Minimum Amount Committed			$ 255,600,000		$ 255,600,000
Intersegment Eliminations [Member]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest					8,400,000
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax			(1,987,000)	$ 3,460,000	(5,166,000)	$ 164,000
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest			$ 24,933,000	$ 15,273,000	$ 55,878,000	$ 25,540,000