Equity Investments (Details) $ in Millions			12 Months Ended
		Jan. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) MW	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance			$ 118.7
Cash Investments			39.2	$ 7.8	$ 5.4
Equity Earnings in ATC			17.5	22.5	18.5
Equity Investment Ending Balance			161.1	118.7
Income Statement Data [Abstract]
ALLETE's Equity in Net Income			$ 17.5	22.5	18.5
FERC [Member] | Return on Equity Complaint 2 [Member]
Income Statement Data [Abstract]
Proposed Return on Equity			9.70%
Proposed Return on Equity, Including Incentive Adder			10.20%
ATC [Member]
Equity Investments [Line Items]
Ownership Percentage			8.00%
Expected Additional Investment in 2019			$ 8.5
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance			118.7	135.6
Cash Investments			6.2	7.8
Equity Earnings in ATC			17.5	22.5	18.5
Distributed ATC Earnings			(15.2)	(19.3)
Remeasurement of Deferred Income Taxes for Equity Method Investments	[1]		0.0	(27.9)
Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments			0.9	0.0
Equity Investment Ending Balance			128.1	118.7	135.6
Balance Sheet Data [Abstract]
Current Assets			87.2	87.7
Non-Current Assets			4,928.8	4,598.9
Total Assets			5,016.0	4,686.6
Current Liabilities			640.0	767.2
Long-Term Debt			2,014.0	1,790.6
Other Non-Current Liabilities			295.3	240.3
Members' Equity			2,066.7	1,888.5
Total Liabilities and Members' Equity			5,016.0	4,686.6
Income Statement Data [Abstract]
Revenue			690.5	721.6	650.8
Operating Expense			358.7	344.9	322.5
Other Expense			108.3	104.1	95.5
Net Income			223.5	272.6	232.8
ALLETE's Equity in Net Income			$ 17.5	$ 22.5	$ 18.5
Authorized Return on Equity			10.32%
Authorized Return on Equity, Including Incentive Adder			10.82%
ATC [Member] | Subsequent Event [Member]
ALLETE's Investment in ATC [Roll Forward]
Cash Investments		$ 0.4
Nobles 2 [Member]
Equity Investments [Line Items]
Ownership Percentage			49.00%
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Ending Balance			$ 33.0
Nobles 2 [Member] | Minimum [Member]
Income Statement Data [Abstract]
Estimated Project Cost			350.0
Portion of Expected Project Costs			170.0
Expected Tax Equity Financing			60.0
Nobles 2 [Member] | Maximum [Member]
Income Statement Data [Abstract]
Estimated Project Cost			400.0
Portion of Expected Project Costs			200.0
Expected Tax Equity Financing			$ 70.0
Tenaska [Member] | Nobles 2 PPA [Member] | Minnesota Power [Member] | Wind Turbine Generators [Member] | MPUC [Member] | Resource Package [Member]
Equity Investments [Line Items]
Generating Capacity to be Counterparty Owned (MW) | MW	[2]		250
Contract Term (Years)			20 years

[1] Impact of the remeasurement of deferred income tax assets and liabilities resulting from the TCJA.

[2] The agreement includes a fixed capacity charge and energy prices that escalate at a fixed rate annually over the term.