Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions		Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Cash, Cash Equivalents and Restricted Cash [Abstract]
Cash and Cash Equivalents		$ 62.5	$ 44.3	$ 25.7	$ 69.3
Restricted Cash included in Prepayments and Other		1.6	0.8	7.3	2.8
Restricted Cash included in Other Non-Current Assets		3.2	20.1	4.4	20.4
Cash, Cash Equivalents and Restricted Cash on the Consolidated Statement of Cash Flows		67.3	65.2	37.4	92.5
Inventories – Net [Abstract]
Fuel	[1]	17.2	23.1
Materials and Supplies		53.7	51.1
Construction of Wind Energy Facilities	[2]	17.2	0.0
Total Inventories – Net		88.1	74.2
Other Non-Current Assets [Abstract]
Contract Assets	[3]	24.1	25.5
Operating Lease, Right-of-Use Asset		18.9	22.4
ALLETE Properties		18.3	18.2
Restricted Cash, Non-Current		3.2	20.1	$ 4.4	$ 20.4
Other Postretirement Benefit Plans		35.2	34.2
Other		83.0	86.4
Total Other Non-Current Assets		182.7	206.8
Other Current Liabilites [Abstract]
PSAs		12.6	12.5
Fuel Adjustment Clause		3.7	3.7
Operating Lease Liabilities		5.6	5.9
Other		38.5	44.6
Total Other Current Liabilities		60.4	66.7
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligations	[4]	170.6	166.6
PSAs		45.8	52.1
Operating Lease Liabilities		13.4	16.5
Other		47.2	50.1
Total Other Non-Current Liabilities		277.0	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 Outlook – ALLETE Clean Energy.)

[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] 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 June 30, 2021 and December 31, 2020.