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Minority Investments - Additional Information (Detail) (USD $) | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2014 | Mar. 31, 2014 | Sep. 30, 2014 | Mar. 31, 2014
Tres Amigas |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |
Number of commonly interconnected power grids | 3 |  | 3 |
Equity method investment, aggregate cost | $ 5,400,000 |  | $ 5,400,000 |
Percentage of ownership interest hold | 26.00% |  | 26.00% |
Blade Dynamics Ltd. |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |
Cost method investment, original cost | 8,000,000 |  | 8,000,000 |
Percentage of ownership interest hold | 19.00% |  | 19.00% |
Impairment charge on investment | $ 3,500,000 | $ 1,300,000 | $ (3,464,000) | $ 1,300,000

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- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571 + Details
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- Definition The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income. + References Reference 1: 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=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25351-111560 + Details
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