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Commitments, Guarantees and Contingencies - North Dakota Wind Development (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
North Dakota Wind Development [Line Items]
Capital Cost Spent to Date	$ 33.5	$ 27.8
Bison 1 Wind Project [Member]
North Dakota Wind Development [Line Items]
Number of Phases	2
Generating Unit Capacity (MW)	82
Transmission Line Capacity (kV)	230
Total Project Cost	173.8
Transmission Line Length (Miles)	22
MPUC, Updated Rates, Order Date	November 3, 2011
Bison 1 Wind Project, Land Restoration and Upgrades [Member]
North Dakota Wind Development [Line Items]
Expected Capital Cost	3.6
Capital Cost Spent to Date	2.6
Bison 1 Wind Project, Phase 1 [Member]
North Dakota Wind Development [Line Items]
Project Completion Date	2010
Bison 1 Wind Project, Phase 2 [Member]
North Dakota Wind Development [Line Items]
Project Completion Date	January 2012
Bison 2 Wind Project [Member]
North Dakota Wind Development [Line Items]
Capital Cost Spent to Date	40.3
MPUC, Project, Approval Date	September 2011
NDPSC, Site Permit, Approval Date	August 2011
Bison 3 Wind Project [Member]
North Dakota Wind Development [Line Items]
Capital Cost Spent to Date	25.8
MPUC, Project, Approval Date	November 2011
NDPSC, Site Permit, Approval Date	October 2011
Bison 2 and 3 Wind Projects, Each [Member]
North Dakota Wind Development [Line Items]
Generating Unit Capacity (MW)	105
Project Completion Date	2012
Expected Capital Cost	$ 160.0
DC Transmission Line from Square Butte [Member]
North Dakota Wind Development [Line Items]
Transmission Line Capacity (kV)	250
Transmission Line Length (Miles)	465