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Equity Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended |  |  |  | 3 Months Ended |
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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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