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Derivative Accounting - Narrative (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Commodity Contracts        
Derivative Accounting        
Additional collateral to counterparties for energy related non-derivative instrument contracts $ 95,000,000   $ 95,000,000  
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
Estimated loss before income taxes to be reclassified from accumulated other comprehensive income     $ 1,000,000  
APS        
Derivative Accounting        
Percentage of unrealized gains and losses on certain derivatives deferred for future rate treatment 100.00%   100.00%