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Minority Investments - Additional Information (Detail) - Tres Amigas $ in Millions | 3 Months Ended | 7 Months Ended
--- | --- | ---
Dec. 31, 2016 USD ($) | Mar. 31, 2016 USD ($) | Jun. 30, 2015 USD ($) | Sep. 30, 2016 USD ($) power_grid
Schedule Of Equity Method Investments [Line Items] |  |  |  |
Number of commonly interconnected power grids | power_grid |  |  |  | 3
Minority investment, aggregate cost |  |  |  | $ 5.4
Impairment of minority interest investments |  |  | $ 0.7 |
Minority investment, percentage of investment sold |  |  |  | 100.00%
Minority share investment proceeds |  | $ 0.3 |  | $ 0.6
Scenario, Forecast |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |
Minority share investment proceeds | $ 0.3 |  |  |
Maximum |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |
Minority share investment proceeds |  |  |  | $ 0.6

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- Definition This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 35 -Paragraph 32 -URI http://asc.fasb.org/extlink&oid=64825282&loc=d3e32787-111569 + Details
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- Definition The cash inflow associated with the sale of equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3179-108585 + Details
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