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Equity Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				3 Months Ended
	Jun. 30, 2013 Tres Amigas [Member]	May 20, 2011 Tres Amigas [Member]	Jan. 06, 2011 Tres Amigas [Member]	Oct. 09, 2009 Tres Amigas [Member] Item	Jun. 30, 2013 Blade Dynamics Ltd. [Member]	Aug. 12, 2010 Blade Dynamics Ltd. [Member]
Schedule of Equity Method Investments [Line Items]
Acquisition date	Oct. 09, 2009				Aug. 12, 2010
Number of commonly interconnected power grids				3
Equity method investment, aggregate cost				$ 1.8
Equity method investment in cash				0.8		8.0
Equity method investment in common stock				1.0
Cash consideration for increase in minority position		$ 1.8	$ 1.8
Equity method investment ownership percentage	26.00%				19.00%

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Cash consideration for increase in minority position

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Equity method investment in cash.

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Equity method investments, number of commonly interconnected power grids.

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Date when the acquirer obtains control of the acquiree, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 25

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This element represents the aggregate cost of investments accounted for under the equity method of accounting.

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-Name Accounting Standards Codification

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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