Regulatory Matters - Great Northern Transmission Line (Details) - Great Northern Transmission Line [Member] $ in Millions	Dec. 31, 2018 USD ($) kV Miles
Regulatory Matters [Line Items]
Total Project Costs Incurred to Date	$ 380.8
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]
Regulatory Matters [Line Items]
Transmission Line Length (Miles) | Miles	220
Transmission Line Capacity (kV) | kV	500
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	$ 203.7
MPUC [Member] | Minnesota Power [Member]
Regulatory Matters [Line Items]
Transmission Line Length (Miles) | Miles	220
Transmission Line Capacity (kV) | kV	500