Operations and Significant Accounting Policies - Balance Sheet and Income Statement Disclosures (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and Cash Equivalents		$ 69.3	$ 69.1	$ 98.9
Restricted Cash, Noncurrent		20.4	8.6	8.6
Restricted Cash included in Prepayments and Other		2.8	1.3	2.6
Cash, Cash Equivalents and Restricted Cash on the Consolidated Statement of Cash Flows		92.5	79.0	110.1	$ 38.3
Accounts Receivable [Abstract]
Billed	[1]	77.2	121.7
Unbilled	[1]	20.1	24.4
Less: Allowance for Doubtful Accounts	[1]	0.9	1.7
Total Accounts Receivable		96.4	144.4
Inventories – Net [Abstract]
Fuel	[2]	25.9	26.0
Materials and Supplies		46.9	44.2
Raw Materials	[3]	0.0	2.8
Work in Progress	[3]	0.0	6.1
Finished Goods	[3]	0.0	8.4
Reserve for Obsolescence	[3]	0.0	(0.8)
Total Inventories		72.8	86.7
Impairment of Long-Lived Assets [Abstract]
Impairment of Real Estate		0.0	0.0	0.0
Indefinite-Lived Intangible Assets [Abstract]
Impairment of Intangible Assets				0.0
Other Non-Current Assets [Abstract]
Contract Asset - Noncurrent	[4]	28.0	30.7
Financing Receivable	[5]	0.0	10.4
Operating Lease, Right-of-Use Asset	[6]	28.6	0.0
Real Estate Investments, Net		21.9	24.4
Restricted Cash, Noncurrent		20.4	8.6	8.6
Other Postretirement Benefit Plans		37.5	0.4
Other		80.8	77.9
Total Other Non-Current Assets		217.2	152.4
Other Current Liabilities [Abstract]
Provision for Interim Rate Refunds	[7]	0.0	40.0
PSAs		12.3	12.6
Contract with Customer, Liability, Current	[8]	0.0	7.6
Provision for Tax Reform Refunds	[9]	0.2	10.7
Business Combination, Contingent Consideration, Liability, Current	[10]	0.0	3.8
Operating Lease, Liability, Current	[11]	6.9	0.0
Other		41.0	53.8
Total Other Current Liabilities		60.4	128.5
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation		160.3	138.6
PSAs		64.6	76.9
Operating Lease, Liability, Noncurrent	[12]	21.8	0.0
Other		46.3	47.1
Total Other Non-Current Liabilities		293.0	262.6
Business Segments [Line Items]
Other – Non-utility		11.6	23.6	23.6
Operating Expenses – Other [Abstract]
Change in Fair Value of Contingent Consideration	[13]	0.0	(2.0)	(0.7)
Total Operating Expenses – Other		$ 0.0	(2.0)	$ (0.7)
Income Taxes [Abstract]
More-Likely-Than-Not Percentage		50.00%		50.00%
U.S. Water Services [Member]
Goodwill [Line Items]
Impairment of Goodwill				$ 0.0
Corporate and Other [Member]
Operating Expenses – Other [Abstract]
Total Operating Expenses – Other	[14]	$ 0.0	$ (2.0)	$ (0.7)

[1] On March 26, 2019, ALLETE sold U.S. Water Services which resulted in the removal of the related accounts receivable from the Consolidated Balance Sheet.

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

[3] On March 26, 2019, ALLETE sold U.S. Water Services which resulted in the removal of the related inventory items from the Consolidated Balance Sheet.

[4] 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.

[5] Finance Receivable related to the 2016 sale of Ormond Crossings and Lake Swamp, which was collected in the second quarter of 2019

[6] See Leases.

[7] Provision for Interim Rate Refund was refunded to Minnesota Power’s retail customers in the second quarter of 2019.

[8] Contract Liabilities consist of deposits received as a result of entering into contracts with our customers prior to completing our performance obligations.

[9] Provision for Tax Reform Refund related to the income tax benefits of the TCJA in 2018 was refunded to Minnesota Power customers in the first quarter of 2019 and is being returned to SWL&P customers through 2020.

[10] Contingent Consideration related to the earnings-based payment resulting from the U.S. Water Services acquisition was paid in the first quarter of 2019.

[11] See Leases.

[12] See Leases.

[13] (a) Contingent Consideration related to the earnings-based payment resulting from the U.S. Water Services acquisition was paid in the first quarter of 2019. (See Note 7. Fair Value.)

[14] See Note 1. Operations and Significant Accounting Policies.