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Summary of Significant Accounting Policies - Additional Information (Detail)
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9 Months Ended |
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Sep. 30, 2014
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Class B Common Stock [Member]
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| Significant Accounting Policies [Line Items] | |
| Common stock contingent convertible basis | One-to-one basis |
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Credit Suisse [Member] | Gulf Wind [Member]
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| Significant Accounting Policies [Line Items] | |
| Description of derivative swap | The counterparty to a 10-year fixed-for-floating swap related to annual electricity generation at the Company's Gulf Wind project. |
| Term of energy derivative instrument acquired | 10 years |