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REGULATORY ENVIRONMENT - RECOVERY OF NATURAL GAS COSTS (Details) $ in Millions |  | 1 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
Mar. 30, 2021 USD ($) | Jul. 31, 2021 USD ($) utility | Feb. 28, 2021 USD ($) | Sep. 30, 2021 USD ($) | Dec. 31, 2020 USD ($)
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Amounts recoverable from customers |  |  |  | $ 139.4 | $ 20.0
Total regulatory assets |  |  |  | 3,628.1 | 3,544.1
Regulatory assets |  |  |  | 3,488.7 | 3,524.1
Energy costs recoverable through rate adjustments |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Total regulatory assets |  |  |  | $ 125.9 | 1.1
MERC extraordinary natural gas costs |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Recovery period of regulatory asset |  |  |  | 27 months |
Total regulatory assets |  |  |  | $ 64.7 | $ 0.0
Illinois Commerce Commission (ICC) | Energy costs recoverable through rate adjustments |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Recovery period of regulatory asset |  |  | 12 months |  |
WE | Public Service Commission of Wisconsin (PSCW) | Energy costs recoverable through rate adjustments |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Amounts recoverable from customers | $ 54.0 |  |  |  |
Recovery period of regulatory asset | 3 months |  |  |  |
WG | Public Service Commission of Wisconsin (PSCW) | Energy costs recoverable through rate adjustments |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Amounts recoverable from customers | $ 24.0 |  |  |  |
Recovery period of regulatory asset | 3 months |  |  |  |
WPS | Public Service Commission of Wisconsin (PSCW) | Energy costs recoverable through rate adjustments |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Amounts recoverable from customers | $ 28.0 |  |  |  |
Recovery period of regulatory asset | 3 months |  |  |  |
PGL | Illinois Commerce Commission (ICC) | Energy costs recoverable through rate adjustments |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Amounts recoverable from customers |  |  | $ 131.0 |  |
NSG | Illinois Commerce Commission (ICC) | Energy costs recoverable through rate adjustments |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Amounts recoverable from customers |  |  | 10.0 |  |
MERC | Minnesota Public Utilities Commission (MPUC) |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Number of utilities filing a joint proposal | utility |  | 4 |  |  |
MERC | Minnesota Public Utilities Commission (MPUC) | Energy costs recoverable through rate adjustments |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Amounts recoverable from customers |  | $ 10.0 |  |  |
Recovery period of regulatory asset |  | 12 months |  |  |
Total regulatory assets |  |  | $ 75.0 |  |
MERC | Minnesota Public Utilities Commission (MPUC) | MERC extraordinary natural gas costs |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Recovery period of regulatory asset |  | 27 months |  |  |
Regulatory assets |  | $ 65.0 |  |  |

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