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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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Equity method investments, number of commonly interconnected power grids.

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Original amount of cost-method investments at the acquisition date.

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- Definition

This element represents the aggregate cost of investments 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 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.12)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 12

-Article 5

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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

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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



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-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

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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