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LONG-TERM DEBT (Details) | 1 Months Ended | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Sep. 30, 2025 USD ($) | Aug. 31, 2025 USD ($) | Jun. 30, 2025 USD ($) d $ / shares | May 31, 2025 USD ($) | Apr. 30, 2025 USD ($) | Sep. 30, 2025 USD ($) | Sep. 30, 2024 USD ($) | Sep. 30, 2025 USD ($) | Sep. 30, 2024 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  | $ 223,600,000 | $ 204,200,000 | $ 667,400,000 | $ 596,800,000
3.10% WE Debentures due June 1, 2025 | WE |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Repayment of long-term debt |  |  | $ 250,000,000.0 |  |  |  |  |  |
Interest rate on long-term debt |  |  | 3.10% |  |  |  |  |  |
4.15% $500M WE Debentures due 10/15/2030 | WE |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate on long-term debt | 4.15% |  |  |  |  | 4.15% |  | 4.15% |
Proceeds from issuance of debt | $ 500,000,000.0 |  |  |  |  |  |  |  |
4.70% $175M WG Debentures due 10/1/2030 | WG |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate on long-term debt | 4.70% |  |  |  |  | 4.70% |  | 4.70% |
Proceeds from issuance of debt | $ 175,000,000.0 |  |  |  |  |  |  |  |
5.39% $125M WG Debentures due 10/1/2035 | WG |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate on long-term debt | 5.39% |  |  |  |  | 5.39% |  | 5.39% |
Proceeds from issuance of debt | $ 125,000,000.0 |  |  |  |  |  |  |  |
3.53% $200M WG Debentures due 09/30/2025 | WG |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Repayment of long-term debt | $ 200,000,000.0 |  |  |  |  |  |  |  |
Interest rate on long-term debt | 3.53% |  |  |  |  | 3.53% |  | 3.53% |
5.20% MERC Senior Notes due 5/1/2030 | MERC |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate on long-term debt |  |  |  |  | 5.20% |  |  |  |
Proceeds from issuance of debt |  |  |  |  | $ 50,000,000.0 |  |  |  |
2.69% MERC Senior Notes due 5/1/2025 | MERC |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Repayment of long-term debt |  |  |  | $ 50,000,000 |  |  |  |  |
Interest rate on long-term debt |  |  |  | 2.69% |  |  |  |  |
5.20% MGU Senior Notes due 5/1/2030 | MGU |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate on long-term debt |  |  |  |  | 5.20% |  |  |  |
Proceeds from issuance of debt |  |  |  |  | $ 75,000,000.0 |  |  |  |
2.69% MGU Senior Notes due 5/1/2025 | MGU |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Repayment of long-term debt |  |  |  | $ 60,000,000 |  |  |  |  |
Interest rate on long-term debt |  |  |  | 2.69% |  |  |  |  |
5.31% $80M UMERC Senior Notes due 8/14/2030 | UMERC |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate on long-term debt |  | 5.31% |  |  |  |  |  |  |
Proceeds from issuance of debt |  | $ 80,000,000.0 |  |  |  |  |  |  |
5.93% $40M UMERC Senior Notes due 8/14/2035 | UMERC |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate on long-term debt |  | 5.93% |  |  |  |  |  |  |
Proceeds from issuance of debt |  | $ 40,000,000.0 |  |  |  |  |  |  |
WEC Energy Group | WEC 3.55% Senior Notes $120M due June 15, 2025 |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Repayment of long-term debt |  |  | $ 120,000,000.0 |  |  |  |  |  |
Interest rate on long-term debt |  |  | 3.55% |  |  |  |  |  |
WEC Energy Group | WEC 5.00% Senior Notes $500M due September 27, 2025 |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Repayment of long-term debt | $ 500,000,000.0 |  |  |  |  |  |  |  |
Interest rate on long-term debt | 5.00% |  |  |  |  | 5.00% |  | 5.00% |
WEC Energy Group | WEC 3.375% Convertible Notes due June 1, 2028 |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate on long-term debt |  |  | 3.375% |  |  |  |  |  |
Proceeds from convertible debt |  |  | $ 900,000,000.0 |  |  |  |  |  |
Principal amount conversion rate applied to |  |  | $ 1,000 |  |  |  |  |  |
Initial conversion ratio |  |  | 7.7901 |  |  |  |  |  |
Initial conversion price, per share | $ / shares |  |  | $ 128.37 |  |  |  |  |  |
Senior Notes | $ 900,000,000.0 |  |  |  |  | $ 900,000,000.0 |  | $ 900,000,000.0 |
Unamortized debt issuance costs | (9,400,000) |  |  |  |  | (9,400,000) |  | (9,400,000) |
Net carrying amount | 890,600,000 |  |  |  |  | 890,600,000 |  | 890,600,000 |
Fair value amount | 928,300,000 |  |  |  |  | 928,300,000 |  | 928,300,000 |
Contractual interest expense |  |  |  |  |  | 7,600,000 | 0 | 9,400,000 | 0
Amortization of debt issuance costs |  |  |  |  |  | 600,000 | 0 | 900,000 | 0
Interest expense |  |  |  |  |  | 8,200,000 | 0 | 10,300,000 | 0
WEC Energy Group | WEC 3.375% Convertible Notes due June 1, 2028 | Early redemption terms |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, redemption price, percentage |  |  | 100.00% |  |  |  |  |  |
WEC Energy Group | WEC 3.375% Convertible Notes due June 1, 2028 | Debt conversion terms one |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Trading days | d |  |  | 20 |  |  |  |  |  |
Consecutive trading days | d |  |  | 30 |  |  |  |  |  |
Threshold percentage of trigger |  |  | 130.00% |  |  |  |  |  |
WEC Energy Group | WEC 3.375% Convertible Notes due June 1, 2028 | Debt conversion terms two |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Trading days | d |  |  | 5 |  |  |  |  |  |
Consecutive trading days | d |  |  | 10 |  |  |  |  |  |
Threshold percentage of trigger |  |  | 98.00% |  |  |  |  |  |
Principal amount conversion rate applied to |  |  | $ 1,000 |  |  |  |  |  |
WEC Energy Group | WEC 3.375% Convertible Notes due June 1, 2028 | Debt conversion terms three |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Trading days prior to redemption | d |  |  | 2 |  |  |  |  |  |
WEC Energy Group | WEC 4.375% Convertible Notes due June 1, 2027 |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Senior Notes | 862,500,000 |  |  |  |  | 862,500,000 |  | 862,500,000 |
Unamortized debt issuance costs | (5,500,000) |  |  |  |  | (5,500,000) |  | (5,500,000) |
Net carrying amount | 857,000,000.0 |  |  |  |  | 857,000,000.0 |  | 857,000,000.0 |
Fair value amount | 1,034,600,000 |  |  |  |  | 1,034,600,000 |  | 1,034,600,000 |
Contractual interest expense |  |  |  |  |  | 9,400,000 | 9,400,000 | 28,300,000 | 12,900,000
Amortization of debt issuance costs |  |  |  |  |  | 800,000 | 800,000 | 2,500,000 | 1,100,000
Interest expense |  |  |  |  |  | 10,200,000 | 10,200,000 | 30,800,000 | 14,000,000.0
WEC Energy Group | WEC 4.375% Convertible Notes due June 1, 2029 |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Senior Notes | 862,500,000 |  |  |  |  | 862,500,000 |  | 862,500,000 |
Unamortized debt issuance costs | (7,300,000) |  |  |  |  | (7,300,000) |  | (7,300,000) |
Net carrying amount | 855,200,000 |  |  |  |  | 855,200,000 |  | 855,200,000 |
Fair value amount | $ 1,056,600,000 |  |  |  |  | 1,056,600,000 |  | 1,056,600,000 |
Contractual interest expense |  |  |  |  |  | 9,400,000 | 9,400,000 | 28,300,000 | 12,900,000
Amortization of debt issuance costs |  |  |  |  |  | 500,000 | 500,000 | 1,500,000 | 700,000
Interest expense |  |  |  |  |  | $ 9,900,000 | $ 9,900,000 | $ 29,800,000 | $ 13,600,000

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