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Regulatory Matters - Great Northern Transmission Line (Details) - Great Northern Transmission Line [Member]
$ in Millions
Dec. 31, 2017
USD ($)
kV
Miles
Regulatory Matters [Line Items]  
Transmission Line Length (Miles) | Miles 220
Transmission Line Capacity (kV) | kV 500
Total Project Costs to Date - Incurred $ 152.4
Minimum [Member]  
Regulatory Matters [Line Items]  
Estimated Capital Expenditures, Including Past Expenditures 560.0
Maximum [Member]  
Regulatory Matters [Line Items]  
Estimated Capital Expenditures, Including Past Expenditures 710.0
Minnesota Power [Member] | Minimum [Member]  
Regulatory Matters [Line Items]  
Estimated Capital Expenditures, Including Past Expenditures 300.0
Minnesota Power [Member] | Maximum [Member]  
Regulatory Matters [Line Items]  
Estimated Capital Expenditures, Including Past Expenditures 350.0
Manitoba Hydro [Member]  
Regulatory Matters [Line Items]  
Total Project Costs to Date - Recovered $ 67.6
MPUC [Member] | Certificate of Need and Route Permit [Member] | Minnesota Power [Member]  
Regulatory Matters [Line Items]  
Transmission Line Length (Miles) | Miles 220
Transmission Line Capacity (kV) | kV 500