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Derivative Accounting - Narrative (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Derivative Accounting          
Amounts reclassified from accumulated other comprehensive income in next twelve months       $ 0  
Commodity Contracts          
Derivative Accounting          
Aggregate fair value of derivative instruments in a net liability position $ 1,595,000 $ 1,595,000   1,595,000  
Additional collateral to counterparties for energy related non-derivative instrument contracts $ 87,000,000 87,000,000   87,000,000  
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          
Aggregate fair value of derivative instruments in a net liability position $ 110,000,000 $ 110,000,000   $ 110,000,000  
Risk Management Assets | Credit Concentration Risk | Two Counterparties          
Derivative Accounting          
Concentration risk 35.00%