Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Recognition of Right-of-use Assets and Lease Liabilities		$ 0.0	$ 30.4
Cash, Cash Equivalents and Restricted Cash [Abstract]
Cash and Cash Equivalents		79.0	100.3	$ 69.3	$ 69.1
Restricted Cash included in Prepayments and Other		3.2	3.4	2.8	1.3
Restricted Cash, Non-Current		2.9	31.5	20.4	8.6
Total Cash, Cash Equivalents and Restricted Cash		85.1	135.2	92.5	79.0
Inventories – Net [Abstract]
Fuel	[1]	25.1		25.9
Materials and Supplies		49.7		46.9
Total Inventories – Net		74.8		72.8
Other Non-Current Assets [Abstract]
Contract Assets	[2]	26.1		28.0
Operating Lease, Right-of-Use Asset		24.0		28.6
ALLETE Properties		21.2		21.9
Restricted Cash, Non-Current		2.9	$ 31.5	20.4	$ 8.6
Other Postretirement Benefit Plans		38.9		37.5
Other		83.1		81.8
Total Other Non-Current Assets		196.2		218.2
Other Current Liabilites [Abstract]
PSAs		12.5		12.3
Provision for Interim Rate Refund	[3]	5.2		0.0
Public Utilities, Fuel Adjustment Clause	[4]	10.3		0.0
Operating Lease Liabilities		6.3		6.9
Other		36.4		41.2
Total Other Current Liabilities		70.7		60.4
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation		164.4		160.3
PSAs		55.2		64.6
Operating Lease Liabilities		17.7		21.8
Other		48.2		46.3
Total Other Non-Current Liabilities		$ 285.5		$ 293.0

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

[2] 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.

[3] Provision for Interim Rate Refund represents reserves for interim rates resulting from the MPUC’s approval of Minnesota Power’s petition and proposal to resolve and withdraw its rate case. (See Note 2. Regulatory Matters.)

[4] See Note 2. Regulatory Matters.