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Long term debt (Details) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Aug. 31, 2017
Jan. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Nov. 27, 2017
Oct. 30, 2017
Aug. 21, 2017
Debt Instrument [Line Items]                
Proceeds from Issuance of First Mortgage Bond     $ 225          
Repayments of Debt     150          
Debt Instrument, Face Amount           $ 75 $ 25 $ 50
Long-term Pollution Control Bond     $ 21          
Debt Instrument, Interest Rate, Stated Percentage     3.98%          
Proceeds from Issuance of Long-term Debt $ 75 $ 150 $ 225 $ 290 $ 145      
First mortgage Bonds - minimum rate     2.51%          
Unsecured Debt, Current     $ 0 $ 150        
First Mortgage Bonds - maximum rate     9.31%          
Debt, Weighted Average Interest Rate     5.03% 4.86%        
Line of Credit Facility, Interest Rate During Period       1.37%        
Line of Credit Facility, Interest Rate at Period End     1.87% 5.00%        
Line of Credit Facility, Commitment Fee Percentage     0.00%          
Pollution Control Revenue Bonds owned by PGE     $ 21 $ 21