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Regulatory Matters Regulatory Matters (Decoupling and Earnings Sharing) (Details) - USD ($)
$ in Thousands
 12 Months Ended 36 Months Ended
Dec. 31, 2017
Dec. 31, 2015
Dec. 31, 2016
Schedule of Decoupling and Earnings Sharing [Line Items]      
Regulatory Assets $ 664,149   $ 699,828
Regulatory Liabilities (848,353)   (396,952)
Decoupling [Member]      
Schedule of Decoupling and Earnings Sharing [Line Items]      
Regulatory Assets 22,359   43,126
Revenue Subject to Refund [Member]      
Schedule of Decoupling and Earnings Sharing [Line Items]      
Regulatory Liabilities (5,770)   (10,297)
Decoupling [Member]      
Schedule of Decoupling and Earnings Sharing [Line Items]      
Regulatory Liabilities (5,816)   (2,405)
OREGON | Decoupling [Member]      
Schedule of Decoupling and Earnings Sharing [Line Items]      
Regulatory Assets     2,021
OREGON | Revenue Subject to Refund [Member]      
Schedule of Decoupling and Earnings Sharing [Line Items]      
Regulatory Liabilities 0   0
OREGON | Decoupling [Member]      
Schedule of Decoupling and Earnings Sharing [Line Items]      
Regulatory Liabilities (1,168)    
IDAHO      
Schedule of Decoupling and Earnings Sharing [Line Items]      
Public Utilities, Approved Return on Equity, Percentage   9.80%  
IDAHO | Decoupling [Member]      
Schedule of Decoupling and Earnings Sharing [Line Items]      
Regulatory Assets 3,471   8,292
IDAHO | Revenue Subject to Refund [Member]      
Schedule of Decoupling and Earnings Sharing [Line Items]      
Regulatory Liabilities $ (2,350)   (5,184)
WASHINGTON      
Schedule of Decoupling and Earnings Sharing [Line Items]      
Decoupling Maximum Rate Increase Request 0.00%    
WASHINGTON | Decoupling [Member]      
Schedule of Decoupling and Earnings Sharing [Line Items]      
Regulatory Assets $ 14,240   30,408
WASHINGTON | Revenue Subject to Refund [Member]      
Schedule of Decoupling and Earnings Sharing [Line Items]      
Regulatory Liabilities $ (3,420)   $ (5,113)