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Equity Investments - Investment in Nobles 2 (Details) $ in Millions | 1 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jul. 31, 2020 USD ($) | Jun. 30, 2020 USD ($) MW | Jun. 30, 2019 USD ($) | Dec. 31, 2019 USD ($)
Investment in Nobles 2 [Line Items] |  |  |  |
Equity Investment Balance |  | $ 267.1 |  | $ 197.6
Cash Investments |  | $ 66.6 | $ 4.6 |
Nobles 2 [Member] |  |  |  |
Investment in Nobles 2 [Line Items] |  |  |  |
Ownership Percentage |  | 49.00% |  |
Equity Investment Balance |  | $ 121.5 |  | $ 56.0
Cash Investments |  | 65.5 |  |
Expected Additional Investment in 2020 |  | $ 35.0 |  |
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 |  |
Subsequent Event [Member] | Nobles 2 [Member] |  |  |  |
Investment in Nobles 2 [Line Items] |  |  |  |
Cash Investments | $ 15.0 |  |  |

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