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Commitments, Guarantees and Contingencies - Transmission (Details) $ in Millions	Mar. 31, 2018 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 Incurred to Date	$ 211.4
Great Northern Transmission Line [Member] | Minimum [Member]
Transmission [Line Items]
Total Project Cost in the U.S.	560.0
Great Northern Transmission Line [Member] | Maximum [Member]
Transmission [Line Items]
Total Project Cost in the U.S.	710.0
Manitoba Hydro [Member] | Great Northern Transmission Line [Member]
Transmission [Line Items]
Project Costs Recovered from Counterparty	110.0
Minnesota Power [Member] | Great Northern Transmission Line [Member] | Minimum [Member]
Transmission [Line Items]
Total Project Cost in the U.S.	300.0
Minnesota Power [Member] | Great Northern Transmission Line [Member] | Maximum [Member]
Transmission [Line Items]
Total Project Cost in the U.S.	$ 350.0
Manitoba Hydro PPA [Member] | Manitoba Hydro [Member]
Transmission [Line Items]
Output Being Purchased (MW) | MW	250

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- Definition

Capital cost spent to date for project.

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Capital Costs Spent To Date, Recovered

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The estimated amount of certain capital expenditures related to environmental compliance, regulatory matters, or other commitments and/or contingencies. This estimate includes expenditures expected in future periods and expenditures made through the end of the current period.

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- Definition

Share of plant output being purchased in megawatts under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

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- Definition

Capacity of a transmission line in kilovolts.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Length of a transmission line in miles.

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