Operations and Significant Accounting Policies - Revenue - Nature of Revenue Streams (Continued) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Payment Terms and Conditions	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] | Industrial Customers [Member] | Long-term Contract with Customer [Member] | Industrial [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected Period to Recognize Minimum Revenue	1 year
Regulated Operations [Member] | Industrial Customers [Member] | Long-term Contract with Customer [Member] | Industrial [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 70
Expected Period to Recognize Minimum Revenue	1 year
Regulated Operations [Member] | Industrial Customers [Member] | Long-term Contract with Customer [Member] | Industrial [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 50
Expected Period to Recognize Minimum Revenue	1 year
Regulated Operations [Member] | Industrial Customers [Member] | Long-term Contract with Customer [Member] | Industrial [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 50
Expected Period to Recognize Minimum Revenue	1 year
Regulated Operations [Member] | Industrial Customers [Member] | Long-term Contract with Customer [Member] | Industrial [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 30
Expected Period to Recognize Minimum Revenue	1 year
Regulated Operations [Member] | Industrial Customers [Member] | Long-term Contract with Customer [Member] | Industrial [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 40
Expected Period to Recognize Minimum Revenue	4 years
Regulated Operations [Member] | Other Power Supplier Customers [Member] | Long-term Contract with Customer [Member] | Industrial [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 70
Regulated Operations [Member] | Other Power Supplier Customers [Member] | Long-term Contract with Customer [Member] | Other Power Suppliers [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 10
Expected Period to Recognize Minimum Revenue	1 year
Regulated Operations [Member] | Other Power Supplier Customers [Member] | Long-term Contract with Customer [Member] | Other Power Suppliers [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 10
Expected Period to Recognize Minimum Revenue	1 year