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Liquidity Facilities and Other Financing Arrangements (Details) - USD ($)
12 Months Ended
Feb. 10, 2012
Dec. 31, 2019
Aug. 02, 2019
Dec. 31, 2018
Oct. 10, 2017
Short-term Debt [Line Items]          
Short-term debt   $ 176,000,000   $ 379,297,000  
Line of Credit Facility, Maximum Borrowing Capacity   1,300,000,000      
Working Capital Needs          
Short-term Debt [Line Items]          
Current borrowing capacity of line of credit         $ 800,000,000.0
Subsidiaries [Member]          
Short-term Debt [Line Items]          
Short-term debt   $ 176,000,000.0   $ 379,297,000  
Weighted-average interest rate on short-term debt (percent)   3.40%      
Line of Credit Facility, Current Same-Day Borrowing Capacity   $ 75,000,000.0      
Line of Credit Facility, Maximum Borrowing Capacity   $ 1,400,000,000 $ 1,000,000,000.0    
Maximum capitalization percentage   65.00%      
Derivative, Basis Spread on Variable Rate   1.25%      
Line of Credit, Unused Capacity, Commitment Fee Percentage   0.175%      
Subsidiaries [Member] | Working Capital Needs          
Short-term Debt [Line Items]          
Current borrowing capacity of line of credit         $ 800,000,000.0
Subsidiaries [Member] | Energy Hedging Activities [Member]          
Short-term Debt [Line Items]          
Outstanding amount for line of credit   $ 1,000,000.0      
Subsidiaries [Member] | Letter of Credit | Working Capital Needs          
Short-term Debt [Line Items]          
Current borrowing capacity of line of credit   2,800,000      
Subsidiaries [Member] | Line of Credit [Member] | Promissory Note with Puget Energy          
Short-term Debt [Line Items]          
Current borrowing capacity of line of credit   $ 30,000,000.0      
Basis spread on variable rate (percent)   25.00%      
Debt instrument variable rate basis   one-month LIBOR      
Revolving Credit Facility [Member]          
Short-term Debt [Line Items]          
Basis spread on variable rate (percent)   1.75%      
Line of Credit Facility, Commitment Fee Percentage 0.275%        
Line of Credit Facility, Fair Value of Amount Outstanding   $ 24,100,000