Operations and Significant Accounting Policies - Revenue - Nature of Revenue Streams (Details)	12 Months Ended
	Dec. 31, 2018 Customers MW
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 or goods provided. 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	50.00%
U.S. Water Services [Member] | Chemicals, Consumable Equipment and Services [Member] | Contract [Member]
Disaggregation of Revenue [Line Items]
Length of Notice Required to Terminate Contract	30-day
Long-term Contract with Customer [Member] | Regulated Operations [Member] | Wholesale Electric Service [Member] | Municipal [Member] | Municipal Customers [Member]
Disaggregation of Revenue [Line Items]
Length of Notice Required to Terminate Contract	three-year
Number of Customers	16
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	four-year
Contract Serving Requirement | MW	10
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	2