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Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
MW
Dec. 31, 2011
Dec. 31, 2010
Square Butte PPA [Member]
     
Power Purchase Agreements [Line Items]      
Generating Unit Capacity (MW) 455    
Output Entitlement 50.00%    
PPA Counterparty Debt Outstanding $ 416.9    
Annual Debt Service 44    
Power Purchased under Long-term Contracts 67.1 61.2 55.2
Interest Expense 11.1 11.1 10.2
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 Charges 0    
Oliver Wind II PPA [Member]
     
Power Purchase Agreements [Line Items]      
Output Being Purchased (MW) 48    
Term of Contract (Years) 25    
Fixed Capacity Charges $ 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    
Transmission Line Capacity (kV) 500