Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions		Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Cash, Cash Equivalents and Restricted Cash [Abstract]
Cash and Cash Equivalents		$ 59.0	$ 44.3	$ 79.0	$ 69.3
Restricted Cash included in Prepayments and Other		4.1	0.8	3.2	2.8
Restricted Cash included in Other Non-Current Assets		2.3	20.1	2.9	20.4
Cash, Cash Equivalents and Restricted Cash on the Consolidated Statement of Cash Flows		65.4	65.2	85.1	92.5
Inventories – Net [Abstract]
Fuel	[1]	12.4	23.1
Materials and Supplies		55.2	51.1
Construction of Wind Energy Facilities	[2]	22.0	0.0
Total Inventories – Net		89.6	74.2
Other Non-Current Assets [Abstract]
Contract Assets	[3]	23.4	25.5
Operating Lease, Right-of-Use Asset		17.7	22.4
ALLETE Properties		17.6	18.2
Restricted Cash, Non-Current		2.3	20.1	$ 2.9	$ 20.4
Other Postretirement Benefit Plans		35.8	34.2
Other		82.9	86.4
Total Other Non-Current Assets		179.7	206.8
Other Current Liabilites [Abstract]
Customer Deposits	[4]	27.5	7.4
PSAs		12.6	12.5
Fuel Adjustment Clause		2.6	3.7
Operating Lease Liabilities		5.0	5.9
Other		36.3	37.2
Total Other Current Liabilities		84.0	66.7
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation	[5]	170.9	166.6
PSAs		42.6	52.1
Operating Lease Liabilities		12.7	16.5
Other		51.9	50.1
Total Other Non-Current Liabilities		278.1	285.3
BNI Energy
Business Segments [Line Items]
Other Receivables		$ 25.0	$ 25.0

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

[2] Project costs related to ALLETE Clean Energy’s Northern Wind and Red Barn wind projects which are expected be sold in late 2022. (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 and Red Barn wind projects which are expected be sold in late 2022. (See Inventories – Net.)

[5] 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 $25.0 million in Other Non-Current Assets on the Consolidated Balance Sheet as of September 30, 2021, and December 31, 2020.