Regulatory Matters - Great Northern Transmission Line (GNTL) (Details) - Great Northern Transmission Line [Member] - Transmission Line [Member]	Sep. 30, 2016 kV Miles
Regulatory Matters [Line Items]
Transmission Line Length (Miles) | Miles	220
Transmission Line Capacity (kV) | kV	500
MPUC [Member] | Great Northern Transmission Line Route Permit [Member] | Minnesota Power [Member]
Regulatory Matters [Line Items]
Transmission Line Length (Miles) | Miles	220
Transmission Line Capacity (kV) | kV	500