Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 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	$ 34.0
Cash, Cash Equivalents and Restricted Cash [Abstract]
Cash and Cash Equivalents		67.0	353.3	$ 69.3	$ 69.1
Restricted Cash included in Prepayments and Other		6.1	7.2	2.8	1.3
Restricted Cash, Non-Current		5.8	4.7	20.4	8.6
Total Cash, Cash Equivalents and Restricted Cash		78.9	365.2	92.5	79.0
Inventories – Net [Abstract]
Fuel	[1]	31.4		25.9
Materials and Supplies		47.6		46.9
Total Inventories – Net		79.0		72.8
Other Non-Current Assets [Abstract]
Contract Assets	[2]	27.3		28.0
Operating Lease, Right-of-Use Asset		26.8		28.6
ALLETE Properties		21.5		21.9
Restricted Cash, Non-Current		5.8	$ 4.7	20.4	$ 8.6
Other Postretirement Benefit Plans		38.0		37.5
Other		79.5		81.8
Total Other Non-Current Assets		198.9		218.2
Other Current Liabilites [Abstract]
PSAs		12.4		12.3
Operating Lease Liabilities		6.9		6.9
Other		38.3		41.2
Total Other Current Liabilities		57.6		60.4
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation		163.2		160.3
PSAs		61.5		64.6
Operating Lease Liabilities		20.0		21.8
Other		44.0		46.3
Total Other Non-Current Liabilities		$ 288.7		$ 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.