Accounting Policies, by Policy (Policies)	9 Months Ended
Sep. 30, 2014
Accounting Policies [Abstract]
Comprehensive Income, Policy [Policy Text Block]	Other comprehensive income For the nine months ended September 30, 2014 and 2013, the Company reclassified $107,000 and $124,000, respectively, from other comprehensive income, of which $173,000 and $200,000, respectively, were recorded to reduce interest expense and $66,000 and $88,000, respectively, were recorded against the income tax provision, in the condensed consolidated statements of operations and comprehensive income. For the three months ended September 30, 2014 and 2013, the Company reclassified $35,000 and $40,000, respectively, from other comprehensive income, of which $57,000 and $76,000, respectively, were recorded to reduce interest expense and $22,000 and $25,000, respectively, were recorded against the income tax provision, in the condensed consolidated statements of operations and comprehensive income.
Contract Termination Fee, Policy [Policy Text Block]	Termination fee On March 15, 2013, the Company finalized the agreement with Southern California Edison Company (“Southern California Edison”), by which the G1 and G3 Standard Offer #4 power purchase agreements (“PPAs”) were terminated and a termination fee of $9.0 million was recorded in the first quarter of 2013 in selling and marketing expenses. Under the agreement, the Company will continue to sell power from G2, the third plant of the Mammoth complex, under its existing PPA with Southern California Edison, with the term of the contract extended by an additional six years until early 2027.
Solar Project Sale, Policy [Policy Text Block]	Solar project sale On March 26, 2014, the Company signed an agreement with RET Holdings, LLC to sell the Heber Solar project in Imperial County, California for $35.25 million. The Company received the first payment of $15.0 million during the first quarter of 2014 and the second payment for the remaining $20.25 million was paid in the second quarter of 2014. Due to certain contingencies in the sale agreement, the Company deferred the pre-tax gain of approximately $7.6 million until the contingencies were resolved in the second quarter of 2014.
Exploratory Drilling Costs Capitalization and Impairment, Policy [Policy Text Block]	Write-off of unsuccessful exploration activities Write-off of unsuccessful activities for the nine months ended September, 30, 2014, was $8.1 million. This represents the write-off of exploration costs related to the Company’s exploration activities in the Wister site in California, which the Company determined in the second quarter of 2014 would not support commercial operations.
Business Combinations Policy [Policy Text Block]	Acquisition of interests in Crump Geyser and North Valley Geothermal projects On August 5, 2014, the Company signed a definitive Purchase and Sale Agreement with Alternative Earth Resources Inc. (“AER”), pursuant to which the Company paid $1.5 million in cash and (i) purchased AER's (a) 50% interest in Crump Geyser Company (“CGC”), which holds the rights to the Crump Geyser geothermal project, and (b) rights to the North Valley geothermal project (ii) obtained an option, exercisable over a four-year period, to purchase certain of AER's New Truckhaven geothermal lease. Prior to this transaction, CGC was consolidated by the Company as variable interest entity. As a result of the acquisition of the remaining interest, the Company continues to consolidate Crump, but now as a wholly owned indirect subsidiary, and so the carrying value of the non-controlling interest of CGC of $1.0 million was reclassified to the Company's equity and the difference of $0.2 million between the fair value of the consideration paid and the related carrying value of the nonconrolling interest acquired was recorded within “additional paid-in capital” in the condensed consolidated statement of equity.
Concentration Risk, Credit Risk, Policy [Policy Text Block]	Concentration of credit risk Financial instruments that potentially subject the Company to a concentration of credit risk consist principally of temporary cash investments and accounts receivable. The Company places its temporary cash investments with high credit quality financial institutions located in the United States (“U.S.”) and in foreign countries. At September 30, 2014 and December 31, 2013, the Company had deposits totaling $23,016,000 and $13,805,000, respectively, in seven U.S. financial institutions that were federally insured up to $250,000 per account. At September 30, 2014 and December 31, 2013, the Company’s deposits in foreign countries amounted to approximately $31,279,000 and $56,133,000, respectively. At September 30, 2014 and December 31, 2013, accounts receivable related to operations in foreign countries amounted to approximately $42,914,000 and $32,231,000, respectively. At September 30, 2014 and December 31, 2013, accounts receivable from the Company’s primary customers (as described immediately below) amounted to approximately 54.1% and 35.0% of the Company’s accounts receivable, respectively. Sierra Pacific Power Company and Nevada Power Company (subsidiaries of NV Energy, Inc.) accounted for 14.6% and 15.3% of the Company’s total revenues for the three months ended September 30, 2014 and 2013, respectively, and 16.6% and 17.0% for the nine months ended September 30, 2014 and 2013, respectively. Southern California Edison accounted for 19.8% and 20.9% of the Company’s total revenues for the three months ended September 30, 2014 and 2013, respectively, and 15.2% and 14.9% for the nine months ended September 30, 2014 and 2013, respectively. Hawaii Electric Light Company accounted for 7.3% and 8.7% of the Company’s total revenues for the three months ended September 30, 2014 and 2013, respectively, and 8.8% and 8.9% for the nine months ended September 30, 2014 and 2013, respectively. Kenya Power and Lighting Co. Ltd. accounted for 15.7% and 13.9% of the Company’s total revenues for the three months ended September 30, 2014 and 2013, respectively, and 15.6% and 10.9% for the nine months ended September 30, 2014 and 2013, respectively. The Company performs ongoing credit evaluations of its customers’ financial condition. The Company has historically been able to collect on all of its receivable balances, and accordingly, no provision for doubtful accounts has been made.
New Accounting Pronouncements, Policy [Policy Text Block]	New accounting pronouncements effective in the nine-month period ended September 30, 2014 Presentation of Unrecognized Tax Benefits In July 2013, the Financial Accounting Standards Board (“FASB”) clarified the accounting guidance on presentation of the unrecognized tax benefits when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. The guidance states that an unrecognized tax benefit (or a portion thereof) should be presented in the financial statements as a reduction to a deferred tax asset for a net operating loss carryforward, a similar tax loss, or a tax credit carryforward, except for certain exceptions specified in the guidance. The exceptions include when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward is not available at the reporting date under the tax law of the applicable jurisdiction to reduce any income taxes that would result from the disallowance of a tax position or the tax law of the applicable jurisdiction does not require the entity to use, and the entity does not intend to use, the deferred tax asset for such purpose, the unrecognized tax benefit should be presented in the financial statements as a liability and not be combined with deferred tax assets. The assessment of whether a deferred tax asset is available is based on the unrecognized tax benefit and deferred tax asset that exist at the reporting date and is to be made assuming the disallowance of the tax position at the reporting date. This accounting update is effective for fiscal periods after December 15, 2013. The provision was applied prospectively to all unrecognized tax benefits that exist on January 1, 2014. The adoption of this guidance did have a material impact on the condensed consolidated financial statements. New accounting pronouncements effective in future periods Reporting Discontinued Operations and Disclosures In April 2014, the FASB issued Accounting Standards Update (“ASU”) No. 2014-08, Presentation of Financial Statements (Topic 205) and Property, Plant and Equipment (Topic 360): Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity. The amendment, required to be applied prospectively for reporting periods beginning after December 15, 2014, limits discontinued operations reporting to disposals of components of an entity that represent strategic shifts that have, or will have, a major effect on operations and financial results. The amendment requires expanded disclosures for discontinued operations and also requires additional disclosures regarding disposals of individually significant components that do not qualify as discontinued operations. Early adoption is permitted, but only for disposals (or classifications as held for sale) that have not been reported in financial statements previously issued or available for issuance. This amendment has no impact on our current disclosures, but will in the future if we dispose of any individually significant components of the Company. Revenues from Contracts with Customers In May 2014, the FASB issued ASU 2014-09, Revenues from Contracts with Customers, Topic 606, which was a joint project of the FASB and the International Accounting Standards Board to clarify the principles for recognizing revenue and to develop a common revenue standard for U.S. GAAP and International Financial Reporting Standards. The update provides that an entity should recognize revenue in connection with the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Specifically, an entity is required to apply each of the following steps: (1) identify the contract(s) with the customer; (2) identify the performance obligations in the contracts; (3) determine the transaction price; (4) allocate the transaction price to the performance obligation in the contract; and (5) recognize revenue when (or as) the entity satisfies a performance obligation. The amendments in this update are effective for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. Early adoption is not permitted. The Company is currently evaluating the potential impact, if any, of the adoption of these amendments on its consolidated financial statements.

Accounting Policies, by Policy (Policies)

9 Months Ended

Sep. 30, 2014

Comprehensive Income, Policy [Policy Text Block]

Other comprehensive income

For the nine months ended September 30, 2014 and 2013, the Company reclassified $107,000 and $124,000, respectively, from other comprehensive income, of which $173,000 and $200,000, respectively, were recorded to reduce interest expense and $66,000 and $88,000, respectively, were recorded against the income tax provision, in the condensed consolidated statements of operations and comprehensive income. For the three months ended September 30, 2014 and 2013, the Company reclassified $35,000 and $40,000, respectively, from other comprehensive income, of which $57,000 and $76,000, respectively, were recorded to reduce interest expense and $22,000 and $25,000, respectively, were recorded against the income tax provision, in the condensed consolidated statements of operations and comprehensive income.

Contract Termination Fee, Policy [Policy Text Block]

Termination fee

On March 15, 2013, the Company finalized the agreement with Southern California Edison Company (“Southern California Edison”), by which the G1 and G3 Standard Offer #4 power purchase agreements (“PPAs”) were terminated and a termination fee of $9.0 million was recorded in the first quarter of 2013 in selling and marketing expenses. Under the agreement, the Company will continue to sell power from G2, the third plant of the Mammoth complex, under its existing PPA with Southern California Edison, with the term of the contract extended by an additional six years until early 2027.

Solar Project Sale, Policy [Policy Text Block]

Solar project sale

On March 26, 2014, the Company signed an agreement with RET Holdings, LLC to sell the Heber Solar project in Imperial County, California for $35.25 million. The Company received the first payment of $15.0 million during the first quarter of 2014 and the second payment for the remaining $20.25 million was paid in the second quarter of 2014. Due to certain contingencies in the sale agreement, the Company deferred the pre-tax gain of approximately $7.6 million until the contingencies were resolved in the second quarter of 2014.

Exploratory Drilling Costs Capitalization and Impairment, Policy [Policy Text Block]

Write-off of unsuccessful exploration activities

Write-off of unsuccessful activities for the nine months ended September, 30, 2014, was $8.1 million. This represents the write-off of exploration costs related to the Company’s exploration activities in the Wister site in California, which the Company determined in the second quarter of 2014 would not support commercial operations.

Business Combinations Policy [Policy Text Block]

Acquisition of interests in Crump Geyser and North Valley Geothermal projects

On August 5, 2014, the Company signed a definitive Purchase and Sale Agreement with Alternative Earth Resources Inc. (“AER”), pursuant to which the Company paid $1.5 million in cash and (i) purchased AER's (a) 50% interest in Crump Geyser Company (“CGC”), which holds the rights to the Crump Geyser geothermal project, and (b) rights to the North Valley geothermal project (ii) obtained an option, exercisable over a four-year period, to purchase certain of AER's New Truckhaven geothermal lease. Prior to this transaction, CGC was consolidated by the Company as variable interest entity. As a result of the acquisition of the remaining interest, the Company continues to consolidate Crump, but now as a wholly owned indirect subsidiary, and so the carrying value of the non-controlling interest of CGC of $1.0 million was reclassified to the Company's equity and the difference of $0.2 million between the fair value of the consideration paid and the related carrying value of the nonconrolling interest acquired was recorded within “additional paid-in capital” in the condensed consolidated statement of equity.

Concentration Risk, Credit Risk, Policy [Policy Text Block]

Concentration of credit risk

Financial instruments that potentially subject the Company to a concentration of credit risk consist principally of temporary cash investments and accounts receivable.

The Company places its temporary cash investments with high credit quality financial institutions located in the United States (“U.S.”) and in foreign countries. At September 30, 2014 and December 31, 2013, the Company had deposits totaling $23,016,000 and $13,805,000, respectively, in seven U.S. financial institutions that were federally insured up to $250,000 per account. At September 30, 2014 and December 31, 2013, the Company’s deposits in foreign countries amounted to approximately $31,279,000 and $56,133,000, respectively.

At September 30, 2014 and December 31, 2013, accounts receivable related to operations in foreign countries amounted to approximately $42,914,000 and $32,231,000, respectively. At September 30, 2014 and December 31, 2013, accounts receivable from the Company’s primary customers (as described immediately below) amounted to approximately 54.1% and 35.0% of the Company’s accounts receivable, respectively.

Sierra Pacific Power Company and Nevada Power Company (subsidiaries of NV Energy, Inc.) accounted for 14.6% and 15.3% of the Company’s total revenues for the three months ended September 30, 2014 and 2013, respectively, and 16.6% and 17.0% for the nine months ended September 30, 2014 and 2013, respectively.

Southern California Edison accounted for 19.8% and 20.9% of the Company’s total revenues for the three months ended September 30, 2014 and 2013, respectively, and 15.2% and 14.9% for the nine months ended September 30, 2014 and 2013, respectively.

Hawaii Electric Light Company accounted for 7.3% and 8.7% of the Company’s total revenues for the three months ended September 30, 2014 and 2013, respectively, and 8.8% and 8.9% for the nine months ended September 30, 2014 and 2013, respectively.

Kenya Power and Lighting Co. Ltd. accounted for 15.7% and 13.9% of the Company’s total revenues for the three months ended September 30, 2014 and 2013, respectively, and 15.6% and 10.9% for the nine months ended September 30, 2014 and 2013, respectively.

The Company performs ongoing credit evaluations of its customers’ financial condition. The Company has historically been able to collect on all of its receivable balances, and accordingly, no provision for doubtful accounts has been made.

New Accounting Pronouncements, Policy [Policy Text Block]

New accounting pronouncements effective in the nine-month period ended September 30, 2014

Presentation of Unrecognized Tax Benefits

In July 2013, the Financial Accounting Standards Board (“FASB”) clarified the accounting guidance on presentation of the unrecognized tax benefits when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. The guidance states that an unrecognized tax benefit (or a portion thereof) should be presented in the financial statements as a reduction to a deferred tax asset for a net operating loss carryforward, a similar tax loss, or a tax credit carryforward, except for certain exceptions specified in the guidance. The exceptions include when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward is not available at the reporting date under the tax law of the applicable jurisdiction to reduce any income taxes that would result from the disallowance of a tax position or the tax law of the applicable jurisdiction does not require the entity to use, and the entity does not intend to use, the deferred tax asset for such purpose, the unrecognized tax benefit should be presented in the financial statements as a liability and not be combined with deferred tax assets. The assessment of whether a deferred tax asset is available is based on the unrecognized tax benefit and deferred tax asset that exist at the reporting date and is to be made assuming the disallowance of the tax position at the reporting date. This accounting update is effective for fiscal periods after December 15, 2013. The provision was applied prospectively to all unrecognized tax benefits that exist on January 1, 2014. The adoption of this guidance did have a material impact on the condensed consolidated financial statements.

New accounting pronouncements effective in future periods

Reporting Discontinued Operations and Disclosures

In April 2014, the FASB issued Accounting Standards Update (“ASU”) No. 2014-08, Presentation of Financial Statements (Topic 205) and Property, Plant and Equipment (Topic 360): Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity. The amendment, required to be applied prospectively for reporting periods beginning after December 15, 2014, limits discontinued operations reporting to disposals of components of an entity that represent strategic shifts that have, or will have, a major effect on operations and financial results. The amendment requires expanded disclosures for discontinued operations and also requires additional disclosures regarding disposals of individually significant components that do not qualify as discontinued operations. Early adoption is permitted, but only for disposals (or classifications as held for sale) that have not been reported in financial statements previously issued or available for issuance. This amendment has no impact on our current disclosures, but will in the future if we dispose of any individually significant components of the Company.

Revenues from Contracts with Customers

In May 2014, the FASB issued ASU 2014-09, Revenues from Contracts with Customers, Topic 606, which was a joint project of the FASB and the International Accounting Standards Board to clarify the principles for recognizing revenue and to develop a common revenue standard for U.S. GAAP and International Financial Reporting Standards. The update provides that an entity should recognize revenue in connection with the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Specifically, an entity is required to apply each of the following steps: (1) identify the contract(s) with the customer; (2) identify the performance obligations in the contracts; (3) determine the transaction price; (4) allocate the transaction price to the performance obligation in the contract; and (5) recognize revenue when (or as) the entity satisfies a performance obligation. The amendments in this update are effective for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. Early adoption is not permitted. The Company is currently evaluating the potential impact, if any, of the adoption of these amendments on its consolidated financial statements.

- Definition

Disclosure of accounting policy for completed business combinations (purchase method, acquisition method or combination of entities under common control). This accounting policy may include a general discussion of the purchase method or acquisition method of accounting (including for example, the treatment accorded contingent consideration, the identification of assets and liabilities, the purchase price allocation process, how the fair values of acquired assets and liabilities are determined) and the entity's specific application thereof. An entity that acquires another entity in a leveraged buyout transaction generally discloses the accounting policy followed by the acquiring entity in determining the basis used to value its interest in the acquired entity, and the rationale for that accounting policy.

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- Definition

Disclosure of accounting policy for credit risk.

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