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GOODWILL AND INTANGIBLES - INTANGIBLE LIABILITIES (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024
Finite-Lived Intangible Liabilities |  |  |  |  |
Net carrying amount | $ 595.0 |  | $ 595.0 |  | $ 566.8
Amortization | 14.7 | $ 13.4 | $ 43.4 | $ 40.2 |
Period of amortization |  |  | 5 years |  |
Amortization to be recorded as an increase to operating revenues |  |  |  |  |
Amortization to be recorded in the next five years |  |  |  |  |
2025 | 57.9 |  | $ 57.9 |  |
2026 | 59.9 |  | 59.9 |  |
2027 | 59.9 |  | 59.9 |  |
2028 | 59.9 |  | 59.9 |  |
2029 | 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 |  |  |  |  |
2025 | 0.2 |  | 0.2 |  |
2026 | 0.2 |  | 0.2 |  |
2027 | 0.2 |  | 0.2 |  |
2028 | 0.2 |  | 0.2 |  |
2029 | 0.2 |  | 0.2 |  |
WECI |  |  |  |  |
Finite-Lived Intangible Liabilities |  |  |  |  |
Gross carrying amount | 763.1 |  | 763.1 |  | 691.5
Accumulated amortization | (168.1) |  | (168.1) |  | (124.7)
Net carrying amount | 595.0 |  | 595.0 |  | 566.8
PPAs | WECI |  |  |  |  |
Finite-Lived Intangible Liabilities |  |  |  |  |
Gross carrying amount | 751.2 |  | 751.2 |  | 679.6
Accumulated amortization | (162.0) |  | (162.0) |  | (119.3)
Net carrying amount | 589.2 |  | $ 589.2 |  | 560.3
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
Accumulated amortization | (4.8) |  | (4.8) |  | (4.2)
Net carrying amount | $ 2.4 |  | $ 2.4 |  | 3.0
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
Accumulated amortization | (1.3) |  | (1.3) |  | (1.2)
Net carrying amount | $ 3.4 |  | $ 3.4 |  | $ 3.5
Interconnection agreements | Tatanka Ridge and Bishop Hill III |  |  |  |  |
Finite-Lived Intangible Liabilities |  |  |  |  |
Weighted average remaining useful life |  |  | 15 years |  |

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