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Note 1 - Business and Significant Accounting Policies (Details Textual) - USD ($) |  |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
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
Dividends, Common Stock, Cash |  |  |  |  |  |  |  |  |  |  |  | $ 20,500,000 | $ 25,700,000 | $ 12,700,000 |
Common Stock, Dividends, Per Share, Declared |  |  |  |  |  |  |  |  |  |  |  | $ 0.41 | $ 0.52 | $ 0.26 |
Accounts Receivable, Net, Current |  |  | $ 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 |  |  | 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 |  |  | 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 |
Income Tax Expense (Benefit) |  |  | (29,669,000) | $ 11,003,000 | $ 6,369,000 | $ 10,886,000 | 2,450,000 | $ 11,988,000 | $ 7,890,000 | $ 9,509,000 |  | $ (1,411,000) | $ 31,837,000 | $ (15,258,000) |
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 |
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 | $ 0.23 |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 9.50
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] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest Expense |  |  |  |  |  |  |  |  |  |  |  | $ (16,000,000) | $ (12,000,000) | $ (44,000,000) |
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  |  |  |  | $ (5,000,000) | (3,000,000) | (17,000,000) |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 59.30
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 49.80
Minimum [Member] | Power Plants [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment, Useful Life |  |  |  |  |  |  |  |  |  |  |  | 15 years |  |  |
UNITED STATES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash, Cash Equivalents, and Short-term Investments |  |  | 21,200,000 |  |  |  | 72,500,000 |  |  |  |  | $ 21,200,000 | $ 72,500,000 |  |
Foreign Countries [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash, Cash Equivalents, and Short-term Investments |  |  | 32,800,000 |  |  |  | 166,200,000 |  |  |  |  | 32,800,000 | 166,200,000 |  |
Accounts Receivable, Net, Current |  |  | $ 78,100,000 |  |  |  | $ 53,300,000 |  |  |  |  | $ 78,100,000 | $ 53,300,000 |  |

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- Definition Amount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, reduced to the estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(3)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(4)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Accumulated adjustment, net of tax, that results from the process of translating subsidiary financial statements and foreign equity investments into the reporting currency from the functional currency of the reporting entity, net of reclassification of realized foreign currency translation gains or losses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e681-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e637-108580 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 30 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=64808268&loc=d3e32022-110900 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 30 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450520&loc=d3e32583-110901 + Details
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- Definition This element represents the amortization of deferred leasing fees incurred by the lessor and amortized over the term of the lease. Such fees represent (a) costs to originate a lease incurred in transactions with independent third parties that (i) result directly from and are essential to acquire that lease and (ii) would not have been incurred had that leasing transaction not occurred and (b) certain costs directly related to specified activities performed by the lessor for that lease. Those activities are: evaluating the prospective lessee's financial condition; evaluating and recording guarantees, collateral, and other security arrangements; negotiating lease terms; preparing and processing lease documents; and closing the transaction. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 25 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=77902619&loc=d3e40246-112709 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.3) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 + Details
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- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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- Definition Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The amount of cash deposited in financial institutions as of the balance sheet date that is insured by the Federal Deposit Insurance Corporation. + References No definition available. + Details
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- Definition Aggregate dividends declared during the period for each share of common stock outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611 + Details
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- Definition Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption. + References No definition available. + Details
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- Definition For an unclassified balance sheet, the accumulated amortization, as of the reporting date, which represents the periodic charge to earnings of initial direct costs which have been deferred and are being allocated over the lease term in proportion to the recognition of rental income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 25 -Paragraph 16 -URI http://asc.fasb.org/extlink&oid=77902619&loc=d3e40588-112709 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 35 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=77913719&loc=d3e40879-112712 + Details
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- Definition For an unclassified balance sheet, this element represents costs incurred by the lessor that are (a) costs to originate a lease incurred in transactions with independent third parties that (i) result directly from and are essential to acquire that lease and (ii) would not have been incurred had that leasing transaction not occurred and (b) certain costs directly related to specified activities performed by the lessor for that lease. Those activities are: evaluating the prospective lessee's financial condition; evaluating and recording guarantees, collateral, and other security arrangements; negotiating lease terms; preparing and processing lease documents; and closing the transaction. This amount is before considering accumulated amortization representing the periodic charge to earnings to recognize the deferred costs over the term of the related lease. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 25 -Paragraph 16 -URI http://asc.fasb.org/extlink&oid=77902619&loc=d3e40588-112709 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 25 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=77902619&loc=d3e40246-112709 + Details
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- Definition Amount of paid and unpaid common stock dividends declared with the form of settlement in cash. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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