Operations and Significant Accounting Policies - Balance Sheet and Income Statement Disclosures (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts Receivable [Abstract]
Billed		$ 121.7	$ 112.6
Unbilled		24.4	24.6
Less: Allowance for Doubtful Accounts		1.7	2.1
Total Accounts Receivable		144.4	135.1
Inventories – Net [Abstract]
Fuel	[1]	26.0	34.8
Materials and Supplies		44.2	46.5
Raw Materials		2.8	2.8
Work in Progress		6.1	4.2
Finished Goods		8.4	8.3
Reserve for Obsolescence		(0.8)	(0.7)
Total Inventories		86.7	95.9
Impairment of Long-Lived Assets [Abstract]
Impairment of Real Estate		0.0	0.0	$ 0.0
Goodwill [Line Items]
Impairment of Goodwill	[2]	0.0	0.0	3.3
Definite-Lived Intangible Assets [Line Items]
Provision for Interim Rate Refunds		40.0	0.0
Provision for Tax Reform Refunds		10.7	0.0
Indefinite-Lived Intangible Assets [Abstract]
Impairment of Intangible Assets		0.0	0.0	0.0
Other Non-Current Assets [Abstract]
Contract Asset - Noncurrent	[3]	30.7	31.6
Financing Receivable	[4]	10.4	11.0
Other		62.2	65.1
Total Other Non-Current Assets		103.3	107.7
Other Current Liabilities [Abstract]
PSAs		12.6	24.5
Contract with Customer, Liability, Current	[5]	7.6	8.7
Business Combination, Contingent Consideration, Liability, Current	[6]	3.8	0.0
Other		53.8	50.0
Total Other Current Liabilities		128.5	83.2
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation		138.6	122.7
PSAs		76.9	89.5
Contingent Consideration	[7]	0.0	5.4
Other		47.1	49.5
Total Other Non-Current Liabilities		262.6	267.1
Business Segments [Line Items]
Other – Non-utility		23.6	23.6	22.3
Operating Expenses – Other [Abstract]
Impairment of Real Estate		0.0	0.0	0.0
Impairment of Goodwill	[2]	0.0	0.0	3.3
Change in Fair Value of Contingent Consideration	[8]	(2.0)	(0.7)	(13.6)
Total Operating Expenses – Other		$ (2.0)	$ (0.7)	$ (10.3)
Income Taxes [Abstract]
More-Likely-Than-Not Percentage		50.00%		50.00%
Remeasurement of Deferred Income Taxes as a Result of the TCJA [Line Items]
Statutory Federal Income Tax Rate		21.00%	35.00%	35.00%
Deferred Income Tax Expense (Benefit)		$ (15.8)	$ 14.4	$ 19.4
Decrease in Investment in ATC		0.0	(27.9)	0.0
Increase to Regulatory Assets		0.0	80.9	0.0
Increase to Regulatory Liabilities		0.0	393.6	0.0
ALLETE Clean Energy [Member]
Goodwill [Line Items]
Impairment of Goodwill				3.3
Operating Expenses – Other [Abstract]
Impairment of Goodwill				3.3
Total Operating Expenses – Other	[9]	0.0	0.0	3.3
U.S. Water Services [Member]
Goodwill [Line Items]
Impairment of Goodwill		0.0	0.0	0.0
Operating Expenses – Other [Abstract]
Impairment of Goodwill		0.0	0.0	0.0
Corporate and Other [Member]
Operating Expenses – Other [Abstract]
Total Operating Expenses – Other	[9]	$ (2.0)	$ (0.7)	$ (13.6)
Discount Rate [Member] | U.S. Water Services [Member]
Goodwill [Line Items]
Discounted Cash Flow Model Significant Assumptions		0.120
Annual Revenue Growth Rate [Member] | U.S. Water Services [Member] | Minimum [Member]
Goodwill [Line Items]
Discounted Cash Flow Model Significant Assumptions		.060
Annual Revenue Growth Rate [Member] | U.S. Water Services [Member] | Maximum [Member]
Goodwill [Line Items]
Discounted Cash Flow Model Significant Assumptions		.100
Measurement Input, Terminal Growth Rate [Member] | U.S. Water Services [Member]
Goodwill [Line Items]
Discounted Cash Flow Model Significant Assumptions		0.035
Developed Technology and Other [Member] | Minimum [Member]
Definite-Lived Intangible Assets [Line Items]
Useful Life (Years)		4 years
Developed Technology and Other [Member] | Maximum [Member]
Definite-Lived Intangible Assets [Line Items]
Useful Life (Years)		10 years
Customer Relationships [Member]
Definite-Lived Intangible Assets [Line Items]
Useful Life (Years)		19 years

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

[2] See Goodwill and Intangible Assets.

[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] Finance Receivable reflects the remaining balance due from the ALLETE Properties sale of its Ormond Crossings project and Lake Swamp wetland mitigation bank.

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

[6] Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 9. Fair Value.)

[7] Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 9. Fair Value.)

[8] See Note 9. Fair Value.

[9] See Note 1. Operations and Significant Accounting Policies.