Commitments, Guarantees and Contingencies - Transmission (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
CapX2020 [Member]
Transmission [Line Items]
Number of CapX2020 Projects	3
Capital Cost Spent to Date	$ 70.9	$ 48.2
CapX2020 [Member] | Minimum [Member]
Transmission [Line Items]
Estimated Capital Expenditures	100
CapX2020 [Member] | Maximum [Member]
Transmission [Line Items]
Estimated Capital Expenditures	110
CapX2020 Fargo to Monticello Projects [Member]
Transmission [Line Items]
Transmission Line Length (Miles)	238
Transmission Line Capacity (kV)	345
Capital Cost Spent to Date	60.0	37.3
CapX2020 Monticello to St. Cloud Project [Member]
Transmission [Line Items]
Transmission Line Length (Miles)	28
Transmission Line Capacity (kV)	345
CapX2020 Bemidji to Grand Rapids Project [Member]
Transmission [Line Items]
Transmission Line Length (Miles)	70
Transmission Line Capacity (kV)	230
Capital Cost Spent to Date	10.9	10.9
Great Northern Transmission Line [Member]
Transmission [Line Items]
Transmission Line Length (Miles)	240
Transmission Line Capacity (kV)	500
Great Northern Transmission Line [Member] | Minnesota Power [Member]
Transmission [Line Items]
Ownership of GNTL	51.00%
Great Northern Transmission Line [Member] | Manitoba Hydro [Member]
Transmission [Line Items]
Ownership of GNTL	49.00%
Great Northern Transmission Line [Member] | Minimum [Member]
Transmission [Line Items]
Estimated Capital Expenditures	200
Great Northern Transmission Line [Member] | Maximum [Member]
Transmission [Line Items]
Estimated Capital Expenditures	$ 300