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Carty Generating Station (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2016 | Dec. 31, 2015
Property, Plant and Equipment [Line Items] |  |
Public Utilities, Allowance for Funds Used During Construction, Additions | $ 501.0 | $ 424.0
Phase-in Plan, Amount of Costs Deferred for Rate-making Purposes |  | 514.0
Allowance for Funds Used During Construction, Capitalized Interest | 50.0 | 41.0
Malpractice Loss Contingency, Letters of Credit and Surety Bonds | 145.6 |
Construction work-in-progress | 647.0 | $ 545.0
Minimum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Construction work-in-progress | 635.0 |
Maximum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Construction work-in-progress | $ 670.0 |

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