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Regulatory Matters - Electric Rates (Details) - USD ($) $ in Millions					3 Months Ended		9 Months Ended
	Nov. 02, 2023	Nov. 01, 2023	Feb. 28, 2023	Nov. 01, 2021	Sep. 30, 2023	Jun. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2023	Dec. 31, 2022
Regulatory Matters [Line Items]
Regulatory Assets					$ 443.3		$ 443.3			$ 441.0
Regulatory Liabilities					549.3		549.3			526.1
ECO Annual Filing Member | MPUC | Minnesota Power
Regulatory Matters [Line Items]
Revenue from cost recovery riders and other regulatory programs					2.2	$ 1.9
ECO Triennial Filing | MPUC | Minnesota Power
Regulatory Matters [Line Items]
Conservation improvement program goal, next fiscal year									$ 12.5
Energy conservation and optimization plan goal, second fiscal year									12.7
Energy conservation and optimization plan goal, third fiscal year									$ 12.8
Electric Rates | Fuel Adjustment Clause 2022 | MPUC | Minnesota Power
Regulatory Matters [Line Items]
Regulatory Assets										13.0
Electric Rates | Fuel Adjustment Clause 2023 | MPUC | Minnesota Power
Regulatory Matters [Line Items]
Regulatory Liabilities					28.3		28.3
Retail Customers | Electric Rates | Current Cost Recovery Rider | MPUC | Minnesota Power
Regulatory Matters [Line Items]
Revenue from cost recovery riders and other regulatory programs							44.9	$ 19.5
Retail Customers | Electric Rates | 2024 Minnesota General Rate Case | MPUC | Minnesota Power | Subsequent Event
Regulatory Matters [Line Items]
Requested rate increase percent		12.00%
Requested return on equity		10.30%
Requested equity capital structure		53.00%
Annual additional revenue generated from requested final rate increase	$ 64.0	$ 89.0
Annual interim rate increase implementation	60 days
Retail Customers | Electric Rates | 2022 Minnesota General Rate Review | MPUC | Minnesota Power
Regulatory Matters [Line Items]
Revenue from cost recovery riders and other regulatory programs			$ 10.0
Requested rate increase percent				18.00%
Requested return on equity				10.25%
Requested equity capital structure				53.81%
Annual additional revenue generated from requested final rate increase				$ 108.0
Authorized return on equity			9.65%
Return on common equity			52.50%
Additional revenue from base rates			$ 60.0
Customer refund					$ 39.4		$ 39.4			$ 18.4

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