Note 1 - General and Basis of Presentation (Details Textual)			3 Months Ended		12 Months Ended
	Mar. 15, 2017 USD ($)	Dec. 29, 2016 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Goodwill			$ 19,710,000		$ 6,650,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			2,000	$ 3,000
Interest Expense			14,923,000	16,023,000
Income Tax Expense (Benefit)			10,886,000	9,509,000
Accumulated Other Comprehensive Income (Loss), Net of Tax			(7,076,000)		(7,732,000)
Exploration Abandonment and Impairment Expense			0	557,000
Cash, Cash Equivalents, and Short-term Investments			174,138,000	$ 148,498,000	230,214,000	$ 185,919,000
Accounts Receivable, Net, Current			68,920,000		$ 80,807,000
Provision for Doubtful Accounts			$ 0
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Primary Customers [Member]
Concentration Risk, Percentage			60.00%		60.00%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Sierra Pacific Power Company And Nevada Power Company [Member]
Concentration Risk, Percentage			18.80%	23.20%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Kenya Power And Lighting Co LTD [Member]
Concentration Risk, Percentage			14.30%	17.40%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Southern California Public Power Authority [Member]
Concentration Risk, Percentage			9.00%	12.10%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Hyundai (Sarulla Goethermal Power Project) [Member]
Concentration Risk, Percentage			6.00%	9.00%
Maximum [Member]
Cash, FDIC Insured Amount			$ 250,000
UNITED STATES
Cash, Cash Equivalents, and Short-term Investments			52,500,000		$ 72,500,000
Foreign Countries [Member]
Cash, Cash Equivalents, and Short-term Investments			137,400,000		166,200,000
Accounts Receivable, Net, Current			42,200,000		$ 53,300,000
Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax			(600,000)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Interest Expense			(3,000)	$ (4,000)
Income Tax Expense (Benefit)			$ (1,000)	$ (1,000)
Viridity Energy, Inc. [Member]
Business Combination, Consideration Transferred		$ 35,300,000
Business Combination, Contingent Consideration, Liability	$ 12,800,000
Business Combination, Contingent Consideration, Number of Installments	2
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	$ 35,000,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	17 years
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Working Capital	$ 300,000
Goodwill	$ 13,500,000