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Summary of Significant Accounting Policies - Schedule of Percentages of Total Revenues from Significant Customers (Detail) (Total Revenue [Member], Customer Concentration Risk [Member])
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3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
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Sep. 30, 2014
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Sep. 30, 2013
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Sep. 30, 2014
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Sep. 30, 2013
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San Diego Gas and Electric [Member]
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| Concentration Risk [Line Items] | ||||
| Concentrations of credit risk | 23.34% | 14.44% | 26.37% | 15.75% |
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Pacific Gas and Electric [Member]
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| Concentration Risk [Line Items] | ||||
| Concentrations of credit risk | 9.78% | 13.88% | 10.81% | 13.11% |
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NV Energy, Inc. [Member]
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| Concentration Risk [Line Items] | ||||
| Concentrations of credit risk | 9.43% | 12.18% | 10.98% | 11.61% |
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PREPA [Member]
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| Concentration Risk [Line Items] | ||||
| Concentrations of credit risk | 10.90% | 8.15% | 11.04% | 8.01% |
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Pelambres [Member]
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| Concentration Risk [Line Items] | ||||
| Concentrations of credit risk | 12.66% | 0.00% | 4.31% | 0.00% |
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Manitoba [Member] | Hydro [Member]
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| Concentration Risk [Line Items] | ||||
| Concentrations of credit risk | 10.96% | 12.27% | 14.24% | 15.20% |
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Texas [Member] | Electric Reliability Council [Member]
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| Concentration Risk [Line Items] | ||||
| Concentrations of credit risk | 15.17% | 11.90% | 13.40% | 12.32% |