Note 1 - General and Basis of Presentation (Details Textual) $ in Thousands		3 Months Ended			12 Months Ended
	Mar. 22, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jan. 01, 2017 USD ($)	Dec. 31, 2016 USD ($)
Income Tax Expense (Benefit), Total		$ (26,942)		$ 11,004
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance		1,384,885	$ 1,295,700	1,194,674	$ 1,295,700				$ 1,168,272
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		5,000		2,000
Interest Expense, Total		14,344		14,923
Accumulated Other Comprehensive Income (Loss), Net of Tax, Total		(909)	(4,706)	(7,493)	(4,706)				(8,175)
Exploration Abandonment and Impairment Expense		123		$ 0
Accounts Receivable, Net, Current, Total		103,580	110,410		110,410
Provision for Doubtful Accounts		0
Deferred Tax Liabilities, Gross, Noncurrent		$ 48,074	61,961		$ 61,961
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Primary Customers [Member]
Concentration Risk, Percentage		59.00%			57.00%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Sierra Pacific Power Company And Nevada Power Company [Member]
Concentration Risk, Percentage		17.40%		18.80%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Southern California Public Power Authority [Member]
Concentration Risk, Percentage		16.30%		9.00%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Kenya Power and Lighting Co LTD [Member]
Concentration Risk, Percentage		15.10%		14.30%
UNITED STATES
Cash, Cash Equivalents, and Short-term Investments, Total		$ 20,300	21,200		$ 21,200
Foreign Countries [Member]
Cash, Cash Equivalents, and Short-term Investments, Total		47,300	32,800		32,800
Accounts Receivable, Net, Current, Total		73,300	78,100		$ 78,100
Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Total		600
Reclassification out of Accumulated Other Comprehensive Income [Member]
Income Tax Expense (Benefit), Total		(4,000)		$ (1,000)
Interest Expense, Total		$ (9,000)		(3,000)
Migdal Loan [Member]
Debt Instrument, Face Amount	$ 100,000
Debt Instrument, Periodic Payment, Total	4,200
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid	$ 37,000
Debt Instrument, Interest Rate, Stated Percentage	4.80%
Debt Instrument Increase in Stated Interest Rate if Rating is Downgraded to ILA Negative	0.50%
Debt Instrument Increase in Stated Interest Rate Each Additional Downgrade	0.25%
Debt Instrument Decrease in Stated Interest Rate for Each Rating Upgrade	0.25%
Debt to EBITDA Ratio Threshold for Rate Increase	4.5
Debt Instrument Increase in Stated Interest Rate if Debt to EBITDA Ratio Exceeds Threshold	0.50%
Debt to EBITDA Ratio Requirement	6
Stockholders Equity, Debt Covenant, Minimum Threshold	$ 650,000
Stockholders Equity to Total Assets, Ratio	25.00%
Stockholders Equity, Debt Covenant, Minimum Threshold to Pay Dividends	$ 800,000
Dividends to Net Income, Ratio	50.00%
Migdal Loan [Member] | Maximum [Member]
Debt Instrument, Increase in Stated Interest Rate	1.00%
Migdal Loan [Member] | Minimum [Member]
Debt Instrument, Interest Rate, Stated Percentage	4.80%
Restatement Adjustment [Member]
Income Tax Expense (Benefit), Total			$ 100	118
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance				(1,827)				$ 1,700	(1,735)
Accumulated Other Comprehensive Income (Loss), Net of Tax, Total				$ (417)					$ (443)
Deferred Tax Liabilities, Gross, Noncurrent						$ 1,100	$ 6,100