Regulatory Matters - Great Northern Transmission Line (Details) - Great Northern Transmission Line [Member] $ in Millions	Sep. 30, 2017 USD ($) kV Miles
Regulatory Matters [Line Items]
Transmission Line Length (Miles) | Miles	220
Transmission Line Capacity (kV) | kV	500
Total Project Costs Incurred to Date	$ 66.9
Minimum [Member]
Regulatory Matters [Line Items]
Total Project Cost in the U.S.	560.0
Maximum [Member]
Regulatory Matters [Line Items]
Total Project Cost in the U.S.	710.0
Minnesota Power [Member] | Minimum [Member]
Regulatory Matters [Line Items]
Total Project Cost in the U.S.	300.0
Minnesota Power [Member] | Maximum [Member]
Regulatory Matters [Line Items]
Total Project Cost in the U.S.	350.0
Manitoba Hydro [Member]
Regulatory Matters [Line Items]
Project Costs Recovered from Counterparty	$ 36.8
MPUC [Member] | Minnesota Power [Member]
Regulatory Matters [Line Items]
Transmission Line Length (Miles) | Miles	220
Transmission Line Capacity (kV) | kV	500