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Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) (USD $)	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Great Northern Transmission Line [Member]
Power Purchase Agreements [Line Items]
Transmission Line Capacity (kV)	500
Square Butte PPA [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Debt Outstanding	$ 404,900,000
Annual Debt Service	44,000,000
Power Purchased under Long-term Contracts	33,000,000	33,000,000
Interest Expense	5,300,000	5,600,000
Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Generating Capacity (MW)	455
Output Entitlement	50.00%
Minnkota PPA [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Oliver Wind I PPA [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Term of Contract (Years)	25
Fixed Capacity Carges	0
Oliver Wind II PPA [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	48
Term of Contract (Years)	25
Fixed Capacity Carges	0
Manitoba Hydro PPA (expires April 2015) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	50
Manitoba Hydro PPA (expires April 2022) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MWh)	1,000,000
Manitoba Hydro PPA (expires 2035) [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)	250
Term of Contract (Years)	15
Minimum [Member] | Great Northern Transmission Line [Member]
Power Purchase Agreements [Line Items]
Estimated Capital Expenditures	200,000,000
Maximum [Member] | Great Northern Transmission Line [Member]
Power Purchase Agreements [Line Items]
Estimated Capital Expenditures	$ 400,000,000

X

- Definition

The estimated amount of certain future capital expenditures related to environmental compliance, regulatory matters, or other committments and/or contingencies.

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- Definition

Capacity of a generating facility, unit, or project in megawatts.

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No definition available.

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- Definition

Long-Term Contract for Purchase of Electric Power, Fixed Capacity Carges

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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.

+ References

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- Definition

Share of plant output being purchased in megawatthours 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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No definition available.

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- Definition

Term in years of 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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Capacity of a transmission line in kilovolts.

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- Definition

Under long-term contracts with public utility districts, cooperatives or other organizations, a utility company receives a portion of the output of a production plant constructed and financed by the district or cooperative. The utility has only a nominal or no investment at all in the plant but pays a proportionate part of the plant's costs, including debt service.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 10

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- Definition

The allocable portion of interest included in charges under a long-term contract to purchase electric power.

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Share of plant output being purchased 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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Amount of operating expense for purchased power procured under arrangements extending beyond one year or the normal operating cycle, if longer, by regulated operation.

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-Publisher FASB

-Name Accounting Standards Codification

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