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Equity Investments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended					3 Months Ended
	Jun. 30, 2012	Mar. 31, 2012 Tres Amigas [Member]	Jun. 30, 2012 Tres Amigas [Member]	May 20, 2011 Tres Amigas [Member]	Jan. 06, 2011 Tres Amigas [Member]	Oct. 09, 2009 Tres Amigas [Member]	Jun. 30, 2012 Blade Dynamics Ltd. [Member]	Aug. 12, 2010 Blade Dynamics Ltd. [Member]
Schedule of Equity Method Investments [Line Items]
Number of commonly interconnected power grids	three
Acquisition date		Oct. 09, 2009					Aug. 12, 2010
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%				25.00%

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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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- Definition

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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

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

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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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