Operations and Significant Accounting Policies - Narrative (Details)	3 Months Ended	12 Months Ended
	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($) segment customer entity unit state MW	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Other Income (Expense) - Other [Line Items]
Number of reportable segments | segment		2
Impairment of long-lived assets held-for-use		$ 0	$ 0	$ 0
Residual value of leased asset		3,000,000
Contract asset - noncurrent		18,500,000	21,000,000
Capitalized contract cost, amortization		2,400,000	2,400,000
Net Loss Attributable to Non-Controlling Interest		$ 68,200,000	58,000,000.0	$ 31,400,000
Immaterial Out-of-Period Adjustment
Other Income (Expense) - Other [Line Items]
Net Loss Attributable to Non-Controlling Interest	$ 5,700,000		3,600,000
Large Power Customers | Customer Concentration Risk
Other Income (Expense) - Other [Line Items]
Number of customers | customer		8
Receivables from large power customers		$ 11,200,000	$ 11,300,000
Number of entities | entity		2
Large Power Customers | Customer Concentration Risk | Largest Customer | Consolidated Operating Revenue
Other Income (Expense) - Other [Line Items]
Concentration risk		21.00%	26.00%	28.00%
Nemadji Trail Energy Center
Other Income (Expense) - Other [Line Items]
Estimated capital expenditures, including past expenditures		$ 700,000,000
Boswell Unit 3 | 2021 Integrated Resource Plan | MPUC
Other Income (Expense) - Other [Line Items]
Net book value		220,000,000
Boswell Unit 4 | 2021 Integrated Resource Plan | MPUC
Other Income (Expense) - Other [Line Items]
Net book value		$ 420,000,000
Glen Ullin Energy Center
Other Income (Expense) - Other [Line Items]
Generating capacity subject to tax equity financing | MW		106
South Peak
Other Income (Expense) - Other [Line Items]
Generating capacity subject to tax equity financing | MW		80
Diamond Spring
Other Income (Expense) - Other [Line Items]
Generating capacity subject to tax equity financing | MW		303
Caddo
Other Income (Expense) - Other [Line Items]
Generating capacity subject to tax equity financing | MW		303
New Energy Solar Energy Facilities
Other Income (Expense) - Other [Line Items]
Generating capacity subject to tax equity financing | MW		14
Square Butte PPA | Square Butte Coal-fired Unit
Other Income (Expense) - Other [Line Items]
Output being purchased (MW) | MW		227.5
Square Butte | Square Butte PPA | Square Butte Coal-fired Unit
Other Income (Expense) - Other [Line Items]
Generating capacity counterparty owned (MW) | MW		455
Natural Gas-Fired | Nemadji Trail Energy Center
Other Income (Expense) - Other [Line Items]
Generating capacity jointly owned (MW) | MW		600
Nobles 2
Other Income (Expense) - Other [Line Items]
Ownership percentage		49.00%
Generating capacity subject to tax equity financing | MW		250
Minnesota Power | Square Butte | Square Butte PPA | Square Butte Coal-fired Unit
Other Income (Expense) - Other [Line Items]
Expected output entitlement		50.00%
Minnesota Power | Wind Turbine Generators | Tenaska | Nobles 2 PPA
Other Income (Expense) - Other [Line Items]
Generating capacity counterparty owned (MW) | MW		250
Contract term		20 years
South Shore Energy | Nemadji Trail Energy Center
Other Income (Expense) - Other [Line Items]
Capital cost spent to date		$ 9,000,000
South Shore Energy | Natural Gas-Fired | Nemadji Trail Energy Center
Other Income (Expense) - Other [Line Items]
Expected output entitlement		20.00%
New Energy
Other Income (Expense) - Other [Line Items]
Discount rate - input for goodwill analysis		14.00%
Terminal growth rate - input for goodwill analysis		3.50%
Regulated Operations | Alternative Programs
Other Income (Expense) - Other [Line Items]
Revenue collection period following the annual period it is recognized		24 months
Regulated Operations | Municipal Customers | Wholesale Electric Service | Long-term Contract with Customer | Municipal
Other Income (Expense) - Other [Line Items]
Number of customers | customer		14
Length of notice required to terminate contract		3 years
Regulated Operations | Industrial Customers | Long-term Contract with Customer | Industrial | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Other Income (Expense) - Other [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period		3 years
Revenue, remaining performance obligation, amount		$ 62,000,000
Regulated Operations | Industrial Customers | Long-term Contract with Customer | Industrial | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01
Other Income (Expense) - Other [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period		1 year
Revenue, remaining performance obligation, amount		$ 15,000,000
Regulated Operations | Industrial Customers | Long-term Contract with Customer | Industrial | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01
Other Income (Expense) - Other [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period
Revenue, remaining performance obligation, amount		$ 12,000,000
Regulated Operations | Industrial Customers | Retail Electric Service | Industrial
Other Income (Expense) - Other [Line Items]
Percentage of total regulated utility k wh sales		55.00%
Regulated Operations | Industrial Customers | Retail Electric Service | Long-term Contract with Customer | Industrial
Other Income (Expense) - Other [Line Items]
Large power customer contracts | customer		8
Contract serving requirement | MW		10
Length of notice required to terminate contract		4 years
Regulated Operations | Other Power Supplier Customers | Sale of Energy under PSA | Long-term Contract with Customer | Other Power Suppliers
Other Income (Expense) - Other [Line Items]
Number of customers | customer		1
Regulated Operations | Large Power Customers | Customer Concentration Risk | Largest Customer | Consolidated Operating Revenue
Other Income (Expense) - Other [Line Items]
Concentration risk		32.00%	32.00%	32.00%
ALLETE Clean Energy
Other Income (Expense) - Other [Line Items]
Number of states operated in | state		7
Energy generation owned (MW) | MW		1,200
Corporate and Other | BNI Energy
Other Income (Expense) - Other [Line Items]
Number of customers | unit		2
Corporate and Other | BNI Energy | Square Butte Coal-fired Unit
Other Income (Expense) - Other [Line Items]
Number of customers | unit		1
Electric Rates | Regulated Operations | Minnesota Power | Municipal Customers
Other Income (Expense) - Other [Line Items]
Number of customers | customer		14
Electric Rates | Retail Customers | Regulated Operations | Minnesota Power
Other Income (Expense) - Other [Line Items]
Number of customers | customer		150,000
Electric Rates | Retail Customers | Regulated Operations | SWL&P
Other Income (Expense) - Other [Line Items]
Number of customers | customer		15,000
Natural Gas | Retail Customers | Regulated Operations | SWL&P
Other Income (Expense) - Other [Line Items]
Number of customers | customer		13,000
Water | Retail Customers | Regulated Operations | SWL&P
Other Income (Expense) - Other [Line Items]
Number of customers | customer		10,000