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Equity Investments Investment in Nobles 2 (Details) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 USD ($) MW | Mar. 31, 2018 USD ($) | Dec. 31, 2018 USD ($)
Investment in Nobles 2 [Line Items] |  |  |
Equity Method Investment Balance | $ 154.8 |  | $ 161.1
Return of Capital From Equity Investments | $ 8.3 | $ 0.0 |
Nobles 2 [Member] |  |  |
Investment in Nobles 2 [Line Items] |  |  |
Ownership Percentage | 49.00% |  |
Equity Method Investment Balance | $ 24.8 |  | $ 33.0
Return of Capital From Equity Investments | $ 8.3 |  |
Wind Turbine Generators [Member] | Resource Package [Member] | Tenaska [Member] | MPUC [Member] | Minnesota Power [Member] | Tenaska PPA [Member] |  |  |
Investment in Nobles 2 [Line Items] |  |  |
Generating Capacity Counterparty Owned (MW) | MW | 250 |  |
Long-term Contract for Purchase of Electric Power, Term of Contract (Years) | 20 years |  |

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