Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2020	Jun. 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	$ 31.2
Cash, Cash Equivalents and Restricted Cash [Abstract]
Cash and Cash Equivalents		25.7	203.1	$ 69.3	$ 69.1
Restricted Cash included in Prepayments and Other		7.3	0.9	2.8	1.3
Restricted Cash, Non-Current		4.4	2.6	20.4	8.6
Total Cash, Cash Equivalents and Restricted Cash		37.4	206.6	92.5	79.0
Inventories – Net [Abstract]
Fuel	[1]	33.5		25.9
Materials and Supplies		48.5		46.9
Total Inventories – Net		82.0		72.8
Other Non-Current Assets [Abstract]
Contract Assets	[2]	26.7		28.0
Operating Lease, Right-of-Use Asset		25.4		28.6
ALLETE Properties		21.5		21.9
Restricted Cash, Non-Current		4.4	$ 2.6	20.4	$ 8.6
Other Postretirement Benefit Plans		38.4		37.5
Other		84.7		81.8
Total Other Non-Current Assets		201.1		218.2
Other Current Liabilites [Abstract]
PSAs		12.4		12.3
Provision for Interim Rate Refund	[3]	11.7		0.0
Operating Lease Liabilities		6.6		6.9
Other		34.7		41.2
Total Other Current Liabilities		65.4		60.4
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation		164.8		160.3
PSAs		58.3		64.6
Operating Lease Liabilities		18.8		21.8
Other		46.2		46.3
Total Other Non-Current Liabilities		$ 288.1		$ 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.)