Equity Investments (Details) $ in Millions			12 Months Ended
		Jan. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance			$ 161.1
Cash Investments			37.9	$ 39.2	$ 7.8
Equity Earnings			21.7	17.5	22.5
Equity Investment Ending Balance			197.6	161.1
Income Statement Data [Abstract]
ALLETE's Equity in Net Income			$ 21.7	17.5	22.5
ATC [Member]
Equity Investments [Line Items]
Ownership Percentage			8.00%
Expected Additional Investment in 2020			$ 2.7
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance			128.1	118.7
Cash Investments			6.6	6.2
Equity Earnings			21.7	17.5	22.5
Distributed ATC Earnings			(16.1)	(15.2)
Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments			1.3	0.9
Equity Investment Ending Balance			141.6	128.1	118.7
Balance Sheet Data [Abstract]
Current Assets			84.6	87.2
Non-Current Assets			5,244.3	4,928.8
Total Assets			5,328.9	5,016.0
Current Liabilities			502.6	640.0
Long-Term Debt			2,312.8	2,014.0
Other Non-Current Liabilities			298.9	295.3
Members' Equity			2,214.6	2,066.7
Total Liabilities and Members' Equity			5,328.9	5,016.0
Income Statement Data [Abstract]
Revenue			744.4	690.5	721.6
Operating Expense			373.5	358.7	344.9
Other Expense			110.5	108.3	104.1
Net Income			260.4	223.5	272.6
ALLETE's Equity in Net Income			$ 21.7	17.5	$ 22.5
Authorized Return on Equity			9.88%
Authorized Return on Equity, Including Incentive Adder			10.38%
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%
Expected Additional Investment in 2020			$ 115.0
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance			33.0
Cash Investments			31.3
Equity Investment Ending Balance			56.0	$ 33.0
Nobles 2 [Member] | Minimum [Member]
Income Statement Data [Abstract]
Estimated Project Cost			350.0
Nobles 2 [Member] | Maximum [Member]
Income Statement Data [Abstract]
Estimated Project Cost			400.0
Expected Tax Equity Financing			$ 125.0
Tenaska [Member] | Nobles 2 PPA [Member] | Minnesota Power [Member] | Wind Turbine Generators [Member] | MPUC [Member] | Resource Package [Member]
Equity Investments [Line Items]
Generating Capacity Counterparty Owned (MW) | MW	[1]		250
Contract Term (Years)			20 years

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