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Long term debt (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Debt Instrument [Line Items]      
Repayments of Long-term Debt $ 100 $ 73 $ 186
Debt Instrument, Interest Rate, Stated Percentage 5.6675%    
First mortgage Bonds - minimum rate 3.46%    
First Mortgage Bonds - maximum rate 9.31%    
Debt, Weighted Average Interest Rate 5.84% 5.83%  
Pollution Control Revenue Bonds owned by PGE $ 21 $ 21  
Long-term Pollution Control Bond, Rate 5.00%