Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash, Cash Equivalents and Restricted Cash [Abstract]
Cash and Cash Equivalents		$ 128.0	$ 98.9	$ 104.4	$ 27.5
Restricted Cash included in Prepayments and Other		4.8	2.6	6.5	2.2
Restricted Cash included in Other Non-Current Assets		8.7	8.6	8.6	8.6
Total Cash, Cash Equivalents and Restricted Cash		141.5	110.1	$ 119.5	$ 38.3
Inventories – Net [Abstract]
Fuel	[1]	27.3	34.8
Materials and Supplies		44.0	46.5
Construction of Wind Energy Facility	[2]	66.7	0.0
Raw Materials		3.1	2.8
Work in Progress		5.5	4.2
Finished Goods		9.0	8.3
Reserve for Obsolescence		(0.8)	(0.7)
Total Inventories – Net		154.8	95.9
Other Non-Current Assets [Abstract]
Contract Assets	[3]	31.1	31.6
Finance Receivable		10.4	11.0
Other		62.9	65.1
Total Other Non-Current Assets		104.4	107.7
Other Current Liabilites [Abstract]
Provision for Interim Rate Refund	[4]	36.7	0.0
Contract Liabilities	[5]	22.7	8.7
PSAs		15.4	24.5
Provision for Tax Reform Refunds	[6]	9.2	0.0
Contingent Consideration - Current	[7]	5.7	0.0
Other		55.2	50.0
Total Other Current Liabilities		144.9	83.2
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation		138.3	122.7
PSAs		80.0	89.5
Contingent Consideration - Non-current	[8]	0.0	5.4
Other		47.3	49.5
Total Other Non-Current Liabilities		$ 265.6	$ 267.1

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

[2] On February 28, 2018, Montana-Dakota Utilities exercised its option to purchase the Thunder Spirit II wind energy facility upon completion, resulting in a reclassification of the project costs from Property, Plant and Equipment – Net to Inventories – Net as ALLETE Clean Energy will not own the facility upon completion.

[3] 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.

[4] Provision for Interim Rate Refund is expected to be refunded to Minnesota Power’s regulated retail customers in the first quarter of 2019 and includes $20.8 million of discounts provided to EITE customers that will be offset against interim rate refunds as of September 30, 2018 ($8.6 million as of December 31, 2017). (See Note 6. Regulatory Matters.)

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

[6] Provision for Tax Reform Refund is expected to be refunded to Minnesota Power customers in the first quarter of 2019 and SWL&P customers in 2019 pending the outcome of SWL&P’s rate filing with the PSCW. (See Note 6. Regulatory Matters.)

[7] Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 5. Fair Value.)

[8] Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 5. Fair Value.)