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Derivative Accounting - Narrative (Details) - USD ($)
3 Months Ended
Mar. 31, 2021
Mar. 31, 2021
Mar. 31, 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 $ 11,520,000 11,520,000  
Additional collateral to counterparties for energy related non-derivative instrument contracts $ 86,000,000 86,000,000  
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
APS      
Derivative Accounting      
Percentage of unrealized gains and losses on certain derivatives deferred for future rate treatment 100.00% 100.00%  
Risk Management Assets | Credit Concentration Risk      
Derivative Accounting      
Concentration risk 59.00%    
Aggregate fair value of derivative instruments in a net liability position $ 26,000,000 $ 26,000,000