Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions		Sep. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Dec. 31, 2020
Cash, Cash Equivalents and Restricted Cash [Abstract]
Cash and Cash Equivalents		$ 42.1	$ 45.1	$ 59.0	$ 44.3
Restricted Cash included in Prepayments and Other		4.2	0.3	4.1	0.8
Restricted Cash included in Other Non-Current Assets		2.3	2.3	2.3	20.1
Cash, Cash Equivalents and Restricted Cash on the Consolidated Statement of Cash Flows		48.6	47.7	65.4	65.2
Inventories – Net [Abstract]
Fuel	[1]	36.2	18.7
Materials and Supplies		70.6	56.1
Renewable Energy Facilities Under Development	[2]	365.4	22.9
Total Inventories – Net		472.2	97.7
Other Non-Current Assets [Abstract]
Contract Assets	[3]	21.5	23.3
Operating Lease, Right-of-Use Asset		13.9	16.4
ALLETE Properties		17.6	19.4
Restricted Cash, Non-Current		2.3	2.3	$ 2.3	$ 20.1
Other Postretirement Benefit Plans		66.5	64.8
Other		79.8	86.7
Total Other Non-Current Assets		201.6	212.9
Other Current Liabilites [Abstract]
Customer Deposits	[4]	111.7	27.2
PSAs		6.1	12.6
Manufactured Gas Plant - Current	[5]	17.8	12.8
Fuel Adjustment Clause		0.0	5.0
Operating Lease Liabilities		3.7	4.8
Redeemable Non-Controlling Interest	[6]	0.0	30.6
Other		49.2	40.0
Total Other Current Liabilities		188.5	133.0
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation	[7]	197.2	184.5
PSAs		28.4	39.5
Manufactured Gas Plant - Non-Current	[8]	0.2	5.2
Operating Lease Liabilities		10.0	11.6
Other		33.0	40.0
Total Other Non-Current Liabilities		268.8	280.8
BNI Energy
Business Segments [Line Items]
Other Receivables		$ 28.5	$ 28.5

[1] Fuel consists primarily of coal inventory at Minnesota Power.

[2] Renewable Energy Facilities Under Development consists primarily of project costs related to ALLETE Clean Energy’s Northern Wind, Rock Aetna, and Red Barn wind projects which are expected to be sold in late 2022 and early 2023, respectively. (See Other Current Liabilities.)

[3] Contract Assets consist of payments made to customers as an incentive to execute or extend service agreements. The contract payments are being amortized over the term of the respective agreements as a reduction to revenue.

[4] Primarily related to deposits received by ALLETE Clean Energy for the Northern Wind, Rock Aetna and Red Barn wind projects which are expected to be sold in late 2022 and early 2023, respectively. (See Inventories – Net.)

[5] The manufactured gas plant represents the current liability for remediation of a former manufactured gas plant site located in Superior, Wisconsin, and formerly operated by SWL&P.

[6] Amount reclassified from Non-Controlling Interest in Subsidiaries resulting from the exercise of an option to buy out a non-controlling interest, which was paid in the first quarter of 2022.

[7] The asset retirement obligation is primarily related to our Regulated Operations and is funded through customer rates over the life of the related assets. Additionally, BNI Energy funds its obligation through its cost-plus coal supply agreements for which BNI Energy has recorded a receivable of $28.5 million in Other Non-Current Assets on the Consolidated Balance Sheet as of September 30, 2022, ($28.5 million as of December 31, 2021).

[8] The manufactured gas plant represents the non-current liability for remediation of a former manufactured gas plant site located in Superior, Wisconsin, and formerly operated by SWL&P.