Note 1 - Business and Significant Accounting Policies (Details Textual) - USD ($)				3 Months Ended								12 Months Ended
	May 07, 2018	Mar. 01, 2018	Mar. 15, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2018	Jan. 01, 2015	Dec. 31, 2014
Income Tax Expense (Benefit), Total				$ (28,329,000)	$ 6,224,000	$ 32,765,000	$ 11,004,000	$ 7,356,000	$ 12,109,000	$ 8,515,000	$ 9,079,000		$ 21,664,000	$ 37,059,000	$ (16,057,000)
Deferred Tax Liabilities, Net, Noncurrent				61,961,000				36,411,000					61,961,000	36,411,000	28,989,000
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent, Total				57,337,000									57,337,000
Deferred Tax Assets, Net, Noncurrent				57,337,000									57,337,000
Retained Earnings (Accumulated Deficit), Ending Balance				327,255,000				215,352,000					327,255,000	215,352,000	152,326,000			$ 44,670,000
Dividends, Common Stock, Cash													$ 20,500,000	$ 25,700,000	$ 12.70
Common Stock, Dividends, Per Share, Declared													$ 0.41	$ 0.52	$ 0.26
Accounts Receivable, Net, Current, Total				110,410,000				80,807,000					$ 110,410,000	$ 80,807,000
Provision for Doubtful Accounts													0
Interest Costs Capitalized													7,224,000	3,287,000	$ 4,075,000
Exploration Abandonment and Impairment Expense				1,796,000				303,000	1,294,000	863,000	557,000		1,796,000	3,017,000	1,579,000
Grants Received to Offset Exploration and Development Costs Incurred													0	300,000	800,000
Accumulated Amortization, Debt Issuance Costs				31,000,000				31,100,000					31,000,000	31,100,000
Amortization of Debt Issuance Costs													5,700,000	6,900,000	8,800,000
Write off of Deferred Debt Issuance Cost													600,000	100,000	500,000
Deferred Costs, Leasing, Gross				4,200,000									$ 4,200,000
Lease Payment Term													23 years
Goodwill, Impairment Loss													$ 0	0
Impairment of Long-Lived Assets to be Disposed of													0
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax, Ending Balance				1,400,000				1,200,000					1,400,000	1,200,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax													11,000,000	9,000,000	27,000,000
Interest Expense, Total				12,987,000	11,692,000	14,540,000	14,923,000	15,828,000	17,137,000	18,401,000	16,023,000		$ 54,142,000	$ 67,389,000	$ 72,577,000
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent													35.00%	35.00%	35.00%
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount													42,896	102,793	467,766
Accumulated Other Comprehensive Income (Loss), Net of Tax, Total				(4,706,000)				(8,175,000)					$ (4,706,000)	$ (8,175,000)	$ (8,164,000)			(9,221,000)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest, Total													151,415,000	95,876,000	125,205,000
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Total				68,087,000	$ 27,559,000	$ 11,846,000	$ 39,617,000	26,326,000	$ 14,285,000	$ 24,308,000	$ 31,375,000		147,109,000	96,294,000	124,148,000
Research and Development Arrangement with Federal Government, Customer Funding to Offset Costs Incurred													$ 0	$ 0	$ 0
Retained Earnings [Member]
Common Stock, Dividends, Per Share, Declared													$ 0.41	$ 0.52	$ 0.26
Subsequent Event [Member]
Common Stock, Dividends, Per Share, Declared	$ 0.10	$ 0.23
Subsequent Event [Member] | Accounting Standards Update 2016-16 [Member]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets																$ 9,500,000
Subsequent Event [Member] | Accounting Standards Update 2014-09 [Member] | Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption																24,100,000
Scenario, Forecast [Member]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent												21.00%
Reclassification out of Accumulated Other Comprehensive Income [Member]
Income Tax Expense (Benefit), Total													$ (5,000,000)	$ (3,000,000)	$ (17,000,000)
Interest Expense, Total													$ (16,000,000)	(12,000,000)	(44,000,000)
Viridity Energy, Inc. [Member]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			19 years										19 years
Puna Geothermal Ventures [Member]
Deferred Costs, Leasing, Gross				4,200,000									$ 4,200,000
Lease Payment Term													23 years
Deferred Costs, Leasing, Accumulated Amortization				2,300,000				2,100,000					$ 2,300,000	2,100,000
Amortization of Deferred Leasing Fees													$ 200,000	$ 200,000	200,000
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Primary Customers [Member]
Concentration Risk, Percentage													57.00%	60.00%
Maximum [Member]
Cash, FDIC Insured Amount				250,000									$ 250,000
Finite-Lived Intangible Asset, Useful Life													25 years
Maximum [Member] | Subsequent Event [Member] | Accounting Standards Update 2016-16 [Member]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets																59,300,000
Maximum [Member] | Power Plants [Member]
Property, Plant and Equipment, Useful Life													30 years
Minimum [Member]
Finite-Lived Intangible Asset, Useful Life													13 years
Minimum [Member] | Subsequent Event [Member] | Accounting Standards Update 2016-16 [Member]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets																$ 49,800,000
Minimum [Member] | Power Plants [Member]
Property, Plant and Equipment, Useful Life													15 years
UNITED STATES
Cash, Cash Equivalents, and Short-term Investments, Total				21,200,000				72,500,000					$ 21,200,000	$ 72,500,000
Foreign Countries [Member]
Cash, Cash Equivalents, and Short-term Investments, Total				32,800,000				166,200,000					32,800,000	166,200,000
Accounts Receivable, Net, Current, Total				78,100,000				53,300,000					78,100,000	53,300,000
Restatement Adjustment [Member]
Income Tax Expense (Benefit), Total				1,300,000				4,900,000					23,075,000	5,222,000	(799,000)
Deferred Tax Liabilities, Net, Noncurrent				61,961,000				1,029,000					61,961,000	1,029,000
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent, Total				(24,800,000)									(24,800,000)
Deferred Tax Assets, Net, Noncurrent				37,202,000									37,202,000
Retained Earnings (Accumulated Deficit), Ending Balance				(24,367,000)				(1,292,000)					(24,367,000)	(1,292,000)	3,930,000		$ 3,100,000	3,131,000
Accumulated Other Comprehensive Income (Loss), Net of Tax, Total				$ (392,000)				$ (443,000)					(392,000)	(443,000)	(497,000)			$ (553,000)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest, Total													(23,024,000)	(5,168,000)	855,000
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Total													$ (23,075,000)	$ (5,222,000)	$ 799,000