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Derivative Accounting - Narrative (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Derivative Accounting            
Derivative liability $ 13,358,000 $ 13,358,000   $ 13,358,000   $ 72,132,000
Commodity Contracts            
Derivative Accounting            
Derivative liability 13,358,000 13,358,000   13,358,000   $ 72,132,000
Additional collateral to counterparties for energy related non-derivative instrument contracts 89,000,000 89,000,000   89,000,000    
Aggregate fair value of derivative instruments in a net liability position $ 15,282,000 15,282,000   15,282,000    
Commodity Contracts | Designated as Hedging Instruments            
Derivative Accounting            
Amount reclassified from accumulated other comprehensive income to earnings related to discontinued cash flow hedges   $ 0 $ 0 $ 0 $ 0  
APS            
Derivative Accounting            
Percentage of unrealized gains and losses on certain derivatives deferred for future rate treatment 100.00% 100.00%   100.00%    
Risk Management Assets | Credit Concentration Risk            
Derivative Accounting            
Concentration risk 37.00%          
Aggregate fair value of derivative instruments in a net liability position $ 17,000,000 $ 17,000,000   $ 17,000,000