Note 4 - Unconsolidated Investments (Details Textual) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 04, 2014 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) MWh	Sep. 30, 2016 USD ($)	Dec. 31, 2014 USD ($)	May 23, 2014 USD ($)	May 16, 2014 USD ($)
Number of Commercial Lenders in Funding Consortium								6
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax		$ 618	$ 1,337	$ 271	$ (3,829)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		$ 22,780	14,406	$ 100,757	70,284
Intersegment Eliminations [Member]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest						$ 14,100
Sarulla [Member] | Lenders Consortium [Member]
Senior Notes								$ 1,170,000
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
Number Of Phases Of Construction				3
Power Utilization | MWh				110
Percentage of Required Production Capacity		100.00%		100.00%
Percentage of Required Injection Capacity		85.00%		85.00%
Supply Commitment, Remaining Minimum Amount Committed						$ 255,600
Payments to Acquire Projects		$ 10,500		$ 37,900
Accumulated Cash Contributions to Acquire Projects		49,800		49,800
Sarulla [Member] | Interest Rate Swap [Member]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax		600	1,300	(300)	3,800
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax		5,600		5,600
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		$ 4,800	$ 10,500	$ 2,100	$ (30,000)
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] | Subject to Fixed Interest Rate [Member]
Senior Notes							$ 100,000
Sarulla [Member] | Lenders Consortium [Member] | Subject to Fixed LIBOR Interest Rate [Member]
Senior Notes							$ 1,070,000
Sarulla [Member] | Lenders Consortium [Member] | Subject to Fixed LIBOR Interest Rate [Member] | Interest Rate Swap [Member]
Senior Notes	$ 960,000