REGULATORY ENVIRONMENT - RECOVERY OF NATURAL GAS COSTS (Details) - USD ($) $ in Millions			1 Months Ended
	Jul. 06, 2021	Mar. 30, 2021	Feb. 28, 2021	Jun. 30, 2021	Dec. 31, 2020
Public Utilities, General Disclosures [Line Items]
Amounts recoverable from customers				$ 213.6	$ 20.0
Illinois Commerce Commission (ICC) | Excess Natural Gas Costs
Public Utilities, General Disclosures [Line Items]
Recovery period of regulatory asset			12 months
WE | Public Service Commission of Wisconsin (PSCW) | Excess Natural Gas Costs
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) | Excess Natural Gas Costs
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) | Excess Natural Gas Costs
Public Utilities, General Disclosures [Line Items]
Amounts recoverable from customers		$ 28.0
Recovery period of regulatory asset		3 months
PGL | Illinois Commerce Commission (ICC) | Excess Natural Gas Costs
Public Utilities, General Disclosures [Line Items]
Amounts recoverable from customers			$ 131.0
NSG | Illinois Commerce Commission (ICC) | Excess Natural Gas Costs
Public Utilities, General Disclosures [Line Items]
Amounts recoverable from customers			10.0
MERC | Minnesota Public Utilities Commission (MPUC) | Excess Natural Gas Costs
Public Utilities, General Disclosures [Line Items]
Amounts recoverable from customers			$ 75.0
MERC | Minnesota Public Utilities Commission (MPUC) | Excess Natural Gas Costs - 12 Month Recovery | Subsequent event
Public Utilities, General Disclosures [Line Items]
Amounts recoverable from customers	$ 10.0
Recovery period of regulatory asset	12 months
MERC | Minnesota Public Utilities Commission (MPUC) | Excess Natural Gas Costs - 27 Month Recovery | Subsequent event
Public Utilities, General Disclosures [Line Items]
Amounts recoverable from customers	$ 65.0
Recovery period of regulatory asset	27 months