Note 1 - Business and Significant Accounting Policies 1 (Details Textual) $ / shares in Units, $ in Thousands			3 Months Ended								12 Months Ended
	May 03, 2018 USD ($)	Mar. 15, 2017	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Dividends, Common Stock, Cash											$ 22,400	$ 26,800	$ 20,500
Common Stock, Dividends, Per Share, Declared | $ / shares											$ 0.44	$ 0.53	$ 0.44
Cash, FDIC Insured Amount			$ 12,900				$ 31,300				$ 12,900	$ 31,300
Cash, Uninsured Amount			84,800				93,900				84,800	93,900
Accounts Receivable, after Allowance for Credit Loss, Current, Total			154,525				137,581				154,525	137,581
Interest Costs Capitalized											3,285	3,728	$ 7,224
Exploration Abandonment and Impairment Expense			0	$ 0	$ 0	$ 0	3	$ 0	$ 0	$ 123	0	126	1,796
Accumulated Amortization, Debt Issuance Costs			19,500				21,800				19,500	21,800
Amortization of Debt Issuance Costs											5,400	4,600	5,700
Write off of Deferred Debt Issuance Cost											0	0	600
Impairment of Long-Lived Assets to be Disposed of											0
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax, Ending Balance			$ 1,500				0				1,500	0
Grants Received to Offset Exploration and Development Costs Incurred											$ 0	$ 0	$ 0
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount | shares											360,500	176,400	42,900
Insured Event, Gain (Loss)											$ (0)	$ 4,463	$ (0)
Operating Lease, Weighted Average Discount Rate, Percent			5.00%								5.00%
Finance Lease, Weighted Average Discount Rate, Percent			7.00%								7.00%
Galena 2 Power Purchase Agreement [Member]
Termination Fees											$ 0	$ 5,000	$ 0
Product [Member]
Revenue, Remaining Performance Obligation, Amount			$ 139,300								$ 139,300
Viridity Energy, Inc. [Member]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life		19 years									19 years
Minimum [Member]
Finite-Lived Intangible Asset, Useful Life											13 years
Maximum [Member]
Finite-Lived Intangible Asset, Useful Life											29 years
Power Plants [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life												15 years
Power Plants [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life												30 years
Puna Geothermal Power Plant [Member]
Insured Event, Gain (Loss)	$ 1,100										$ 9,300
Pacific Gas & Electric [Member]
Percentage of Total Revenue											1.50%	1.90%	2.00%
Kenya Power and Lighting Co LTD [Member]
Accounts Receivable, Past Due			$ 40,700								$ 40,700
Proceeds, Overdue Accounts Receivable											$ 24,200
Accounts Receivable, Past Due, Average Number of Days Overdue			70								70
Accounts Receivable, Past Due, Average Number of Days Overdue, Number of Days Increase During Period											10
ENNE [Member]
Accounts Receivable, Past Due			$ 20,100								$ 20,100
Accounts Receivable, Credit Loss Expense (Reversal)											$ 0
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Primary Customers [Member]
Concentration Risk, Percentage											58.00%	56.00%
Non-US [Member]
Accounts Receivable, after Allowance for Credit Loss, Current, Total			$ 118,800				$ 102,000				$ 118,800	$ 102,000