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Commitments, Guarantees and Contingencies - North Dakota Wind Development (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2011
North Dakota Wind Development [Line Items]
Capital Cost Spent to Date	$ 19.7
Bison 1 Wind Project [Member]
North Dakota Wind Development [Line Items]
Number of Phases	2
Generating Unit Capacity (MW)	82
Expected Capital Cost	177
Capital Cost Spent to Date	158.8
MPUC, Current Cost Recovery, Approval Date	2009
MPUC, Rates, Approval Date	July 2010
MPUC, Rates, Effective Date	Aug. 1, 2010
MPUC, Updated Rates, Filing Date	Mar. 31, 2011
Bison 1 Wind Project, Phase 1 [Member]
North Dakota Wind Development [Line Items]
Transmission Line Capacity (kV)	230
Transmission Line Length (Miles)	22
Number of Turbines	16
Capacity of Turbines (MW)	2.3
Project Completion Date	2010
Bison 1 Wind Project, Phase 2 [Member]
North Dakota Wind Development [Line Items]
Number of Turbines	15
Capacity of Turbines (MW)	3.0
Project Completion Date	2011
Bison 2 Wind Project [Member]
North Dakota Wind Development [Line Items]
MPUC, Project, Approval Date	Aug. 24, 2011
NDPSC, Site Permit, Approval Date	Aug. 10, 2011
Bison 3 Wind Project [Member]
North Dakota Wind Development [Line Items]
MPUC, Project, Approval Date	Oct. 12, 2011
NDPSC, Site Permit, Approval Date	Oct. 20, 2011
Bison 2 and 3 Wind Projects [Member]
North Dakota Wind Development [Line Items]
Generating Unit Capacity (MW)	105
Project Completion Date	2012
Expected Capital Cost	$ 160
DC Transmission Line from Square Butte [Member]
North Dakota Wind Development [Line Items]
Transmission Line Capacity (kV)	250
Transmission Line Length (Miles)	465
Transmission Line Purchase Date	December 2009

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Capacity of each turbine in megawatts at a site.

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Capital cost spent to date for project.

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Expected capital cost for each individual project.

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Capacity of each generating unit/individual project in megawatts.

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Date/expected date of approval by the Minnesota Public Utilities Commission (MPUC) of current cost recovery for expenditures and investments related to a project.

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Date/expected date of project approval by the Minnesota Public Utilities Commission (MPUC).

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Date/expected date of approval by the Minnesota Public Utilities Commission (MPUC) establishing rates.

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Effective date of established rates as approved by the Minnesota Public Utilities Commission (MPUC).

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Filing date of petition to the Minnesota Public Utilities Commission (MPUC) seeking updated rates for additional investments and expenditures.

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Date/expected date of approval of site permit by the North Dakota Public Service Commission (NDPSC).

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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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Number of phases for project.

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Number of turbines at a site.

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Date/expected date of project completion.

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Capacity of a transmission line in kilovolts.

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Length of a transmission line in miles.

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Date of purchase of a transmission line.

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