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Minority Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended				6 Months Ended
	Sep. 30, 2013 Tres Amigas	May 20, 2011 Tres Amigas	Jan. 06, 2011 Tres Amigas	Oct. 09, 2009 Tres Amigas Item	Sep. 30, 2013 Blade Dynamics Ltd.	Aug. 12, 2010 Blade Dynamics Ltd.
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%
Cost method investment ownership percentage					19.00%

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Cash consideration for increase in minority position

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Cost method Investment ownership percentage.

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Equity method investment in cash.

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

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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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