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Commitments, Guarantees and Contingencies - Transmission (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
kV
Miles
MW
Great Northern Transmission Line [Member]  
Transmission [Line Items]  
Transmission Line Length (Miles) | Miles 220
Transmission Line Capacity (kV) | kV 500
Total Project Costs to Date - Incurred $ 152.4
Great Northern Transmission Line [Member] | Minimum [Member]  
Transmission [Line Items]  
Estimated Capital Expenditures, Including Past Expenditures 560.0
Great Northern Transmission Line [Member] | Maximum [Member]  
Transmission [Line Items]  
Estimated Capital Expenditures, Including Past Expenditures $ 710.0
Manitoba Hydro [Member] | Manitoba Hydro PPA (expires May 2035) [Member]  
Transmission [Line Items]  
Output Being Purchased (MW) | MW 250
Manitoba Hydro [Member] | Great Northern Transmission Line [Member]  
Transmission [Line Items]  
Total Project Costs to Date - Recovered $ 67.6
Minnesota Power [Member] | Great Northern Transmission Line [Member] | Minimum [Member]  
Transmission [Line Items]  
Estimated Capital Expenditures, Including Past Expenditures 300.0
Minnesota Power [Member] | Great Northern Transmission Line [Member] | Maximum [Member]  
Transmission [Line Items]  
Estimated Capital Expenditures, Including Past Expenditures $ 350.0