Operations and Significant Accounting Policies - Revenue - Nature of Revenue Streams (Details)	12 Months Ended
	Dec. 31, 2020 Customers MW	Dec. 31, 2019 Customers	Dec. 31, 2018 Customers
Disaggregation of Revenue [Line Items]
Large Power Customer Contracts	8
Revenue, Performance Obligation, Description of Payment Terms	Payment terms and conditions vary across our businesses. Aside from taconite-producing Large Power Customers, payment terms generally require payment to be made within 15 to 30 days from the end of the period that the service has been rendered. In the case of its taconite-producing Large Power Customers, as permitted by the MPUC, Minnesota Power requires weekly payments for electric usage based on monthly energy usage estimates.
Regulated Operations [Member] | Alternative Programs [Member]
Disaggregation of Revenue [Line Items]
Revenue Collection Period Following the Annual Period it is Recognized	24 months
Regulated Operations [Member] | Retail Electric Service [Member] | Industrial [Member] | Industrial Customers [Member]
Disaggregation of Revenue [Line Items]
Percentage of Total Regulated Utility kWh Sales	47.00%
Long-term Contract with Customer [Member] | Regulated Operations [Member] | Wholesale Electric Service [Member] | Municipal [Member] | Municipal Customers [Member]
Disaggregation of Revenue [Line Items]
Number of Customers	15
Long-term Contract with Customer [Member] | Regulated Operations [Member] | Retail Electric Service [Member] | Industrial [Member] | Industrial Customers [Member]
Disaggregation of Revenue [Line Items]
Length of Notice Required to Terminate Contract	4 years
Contract Serving Requirement | MW	10
Long-term Contract with Customer [Member] | Regulated Operations [Member] | Retail Electric Service [Member] | Municipal Customers [Member] | Municipal [Member]
Disaggregation of Revenue [Line Items]
Length of Notice Required to Terminate Contract	3 years
Long-term Contract with Customer [Member] | Regulated Operations [Member] | Sale of Energy under PSA [Member] | Other Power Suppliers [Member] | Other Power Supplier Customers [Member]
Disaggregation of Revenue [Line Items]
Number of Customers	1
Customer Concentration Risk [Member] | Large Power Customers [Member]
Disaggregation of Revenue [Line Items]
Number of Customers	8	9	9
Customer Concentration Risk [Member] | Consolidated Operating Revenue [Member] | Large Power Customers [Member] | Largest Customer [Member]
Disaggregation of Revenue [Line Items]
Concentration Risk, Percentage	25.00%	12.00%	10.00%
Customer Concentration Risk [Member] | Consolidated Operating Revenue [Member] | Large Power Customers [Member] | Regulated Operations [Member] | Largest Customer [Member]
Disaggregation of Revenue [Line Items]
Concentration Risk, Percentage	29.00%