Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Recognition of Right-of-use Assets and Lease Liabilities	[1]	$ 30.4	$ 0.0
Cash, Cash Equivalents and Restricted Cash [Abstract]
Cash and Cash Equivalents		100.3	128.0	$ 69.1	$ 98.9
Restricted Cash included in Prepayments and Other		3.4	4.8	1.3	2.6
Restricted Cash included in Other Non-Current Assets		31.5	8.7	8.6	8.6
Total Cash, Cash Equivalents and Restricted Cash		135.2	$ 141.5	79.0	$ 110.1
Inventories – Net [Abstract]
Fuel	[2]	37.8		26.0
Materials and Supplies		46.8		44.2
Raw Materials (b)	[3]	0.0		2.8
Work in Progress (b)	[3]	0.0		6.1
Finished Goods (b)	[3]	0.0		8.4
Reserve for Obsolescence (b)	[3]	0.0		(0.8)
Total Inventories – Net		84.6		86.7
Other Non-Current Assets [Abstract]
Contract Assets	[4]	28.6		30.7
Finance Receivable	[5]	0.0		10.4
Operating Lease, Right-of-Use Asset	[6]	30.4		0.0
ALLETE Properties		22.4		24.4
Other		110.2		86.9
Total Other Non-Current Assets		191.6		152.4
Other Current Liabilites [Abstract]
Provision for Interim Rate Refund	[7]	0.0		40.0
PSAs		12.3		12.6
Contract Liabilities	[8]	0.0		7.6
Provision for Tax Reform Refunds	[9]	0.3		10.7
Contingent Consideration	[10]	0.0		3.8
Operating Lease Liabilities	[11]	7.1		0.0
Other		39.0		53.8
Total Other Current Liabilities		58.7		128.5
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation		157.4		138.6
PSAs		67.7		76.9
Operating Lease Liabilities		23.3		0.0
Other		44.7		47.1
Total Other Non-Current Liabilities		$ 293.1		$ 262.6

[1] See Leases.

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

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

[4] 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.

[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 consisted of deposits received as a result of entering into contracts with customers prior to completing 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 will be refunded to SWL&P customers in 2019 and 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.