Operations and Significant Accounting Policies - Balance Sheet and Income Statement Disclosures (Details) - USD ($)		12 Months Ended
		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and Cash Equivalents		$ 36,400,000	$ 45,100,000	$ 44,300,000
Restricted Cash included in Prepayments and Other		1,500,000	300,000	800,000
Restricted Cash included in Other Non-Current Assets		2,300,000	2,300,000	20,100,000
Cash, Cash Equivalents and Restricted Cash on the Consolidated Statement of Cash Flows		40,200,000	47,700,000	65,200,000	$ 92,500,000
Accounts Receivable [Abstract]
Billed		107,100,000	100,600,000
Unbilled		29,200,000	24,700,000
Less: Allowance for Doubtful Accounts		1,600,000	1,800,000
Total Accounts Receivable		134,700,000	123,500,000
Nontrade Receivables, Current		3,200,000	200,000
Accounts Receivable, Allowance		137,900,000	123,700,000
Inventories – Net [Abstract]
Fuel	[1]	33,400,000	18,700,000
Materials and Supplies		75,100,000	56,100,000
Construction of Wind Energy Facilities	[2]	347,400,000	22,900,000
Total Inventories – Net		455,900,000	97,700,000
Property, Plant and Equipment [Line Items]
Impairment of Long-Lived Assets Held-for-use		0	0
Other Non-Current Assets [Abstract]
Contract Asset - Noncurrent	[3]	21,000,000	23,300,000
Operating Lease, Right-of-Use Asset		12,700,000	16,400,000
ALLETE Properties		19,100,000	19,400,000
Restricted Cash, Noncurrent		2,300,000	2,300,000	$ 20,100,000
Other Postretirement Benefit Plans		58,800,000	64,800,000
Other		90,400,000	86,700,000
Total Other Non-Current Assets		204,300,000	212,900,000
Other Liabilities [Abstract]
Customer Deposits	[4]	150,700,000	27,200,000
PSAs		6,100,000	12,600,000
Manufactured Gas Plant - Current	[5]	14,700,000	12,800,000
Fuel Adjustment Clause		0	5,000,000.0
Operating Lease Liabilities, Current		3,200,000	4,800,000
Public Utilities - Reserve for Interim Rate Refund		18,400,000	0
Redeemable Non-Controlling Interest	[6]	0	30,600,000
Other		57,900,000	40,000,000.0
Total Other Current Liabilities		251,000,000.0	133,000,000.0
Asset Retirement Obligation	[7]	200,400,000	184,500,000
PSAs		26,900,000	39,500,000
Operating Lease Liabilities, Non-Current		9,300,000	11,600,000
Other		32,400,000	45,200,000
Total Other Non-Current Liabilities		$ 269,000,000.0	$ 280,800,000
Income Taxes [Abstract]
More-Likely-Than-Not Percentage		50.00%	50.00%
BNI Energy [Member]
Business Segments [Line Items]
Other Receivables		$ 32,400,000	$ 28,500,000
Taconite Harbor [Member] | Resource Package [Member] | MPUC [Member]
Property, Plant and Equipment [Line Items]
Net Book Value		25,000,000
Boswell Unit 3 [Member] | 2021 Integrated Resource Plan [Member] | MPUC [Member]
Property, Plant and Equipment [Line Items]
Net Book Value		235,000,000
Boswell Unit 4 [Member] | 2021 Integrated Resource Plan [Member] | MPUC [Member]
Property, Plant and Equipment [Line Items]
Net Book Value		$ 435,000,000

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

[2] Project costs related to ALLETE Clean Energy’s Northern Wind project sold in January 2023 and Red Barn wind project which is expected to be sold in the first half 2023. (See Other Current Liabilities.)

[3] Contract Assets include 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 project sold in January 2023 and the Red Barn wind project which is expected to be sold in the first half of 2023. (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. (See Note 9. Commitments, Guarantees and Contingencies.)

[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 $32.4 million in Other Non-Current Assets on the Consolidated Balance Sheet as of December 31, 2022 ($28.5 million as of December 31, 2021).