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GOODWILL AND INTANGIBLES - INTANGIBLE LIABILITIES (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2026 | Jun. 30, 2025 | Jun. 30, 2026 | Jun. 30, 2025 | Dec. 31, 2025
Finite-Lived Intangible Liabilities |  |  |  |  |
Net carrying amount | $ 550.3 |  | $ 550.3 |  | $ 580.3
Amortization | 15.0 | $ 14.8 | $ 30.0 | $ 28.7 |
Period of amortization |  |  | 5 years |  |
Amortization to be recorded as an increase to operating revenues |  |  |  |  |
Amortization to be recorded in the next five years |  |  |  |  |
2026 | 59.9 |  | $ 59.9 |  |
2027 | 59.9 |  | 59.9 |  |
2028 | 59.9 |  | 59.9 |  |
2029 | 59.9 |  | 59.9 |  |
2030 | 59.9 |  | 59.9 |  |
Amortization to be recorded as a decrease to other operation and maintenance |  |  |  |  |
Amortization to be recorded in the next five years |  |  |  |  |
2026 | 0.2 |  | 0.2 |  |
2027 | 0.2 |  | 0.2 |  |
2028 | 0.2 |  | 0.2 |  |
2029 | 0.2 |  | 0.2 |  |
2030 | 0.2 |  | 0.2 |  |
WECI |  |  |  |  |
Finite-Lived Intangible Liabilities |  |  |  |  |
Gross carrying amount | 763.1 |  | 763.1 |  | 763.1
Intangible liabilities, accumulated amortization | (212.8) |  | (212.8) |  | (182.8)
Net carrying amount | 550.3 |  | 550.3 |  | 580.3
PPAs | WECI |  |  |  |  |
Finite-Lived Intangible Liabilities |  |  |  |  |
Gross carrying amount | 751.2 |  | 751.2 |  | 751.2
Intangible liabilities, accumulated amortization | (206.1) |  | (206.1) |  | (176.5)
Net carrying amount | 545.1 |  | $ 545.1 |  | 574.7
PPAs | Blooming Grove, Tatanka Ridge, Jayhawk, Thunderhead, Samson I, Sapphire Sky, Delilah I, and Hardin III |  |  |  |  |
Finite-Lived Intangible Liabilities |  |  |  |  |
Weighted average remaining useful life |  |  | 10 years |  |
Proxy revenue swap | WECI |  |  |  |  |
Finite-Lived Intangible Liabilities |  |  |  |  |
Gross carrying amount | 7.2 |  | $ 7.2 |  | 7.2
Intangible liabilities, accumulated amortization | (5.3) |  | (5.3) |  | (4.9)
Net carrying amount | $ 1.9 |  | $ 1.9 |  | 2.3
Proxy revenue swap | Upstream |  |  |  |  |
Finite-Lived Intangible Liabilities |  |  |  |  |
Weighted average remaining useful life |  |  | 3 years |  |
Length of proxy revenue contract, in years | 10 years |  | 10 years |  |
Interconnection agreements | WECI |  |  |  |  |
Finite-Lived Intangible Liabilities |  |  |  |  |
Gross carrying amount | $ 4.7 |  | $ 4.7 |  | 4.7
Intangible liabilities, accumulated amortization | (1.4) |  | (1.4) |  | (1.4)
Net carrying amount | $ 3.3 |  | $ 3.3 |  | $ 3.3
Interconnection agreements | Tatanka Ridge and Bishop Hill III |  |  |  |  |
Finite-Lived Intangible Liabilities |  |  |  |  |
Weighted average remaining useful life |  |  | 14 years |  |

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