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Financial Derivative Instruments Narrative (Details)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
USD ($)
Sep. 30, 2019
USD ($)
Jun. 30, 2020
USD ($)
Jun. 08, 2020
USD ($)
May 04, 2020
USD ($)
Dec. 31, 2019
Agreements
Jun. 30, 2019
Derivative [Line Items]              
Percentage of actual consumption hedged 95.00%   79.00%        
Debt Instrument, Face Amount       $ 1,800 $ 2,000    
Current Unrealized Net Losses in OCI $ 63   $ 63        
Maximum sum of derivatives of counterparty to be included in other (less than $8 million) $ 8   $ 8        
Cash Collateral Percent Of Fair Value Fuel Derivatives Contracts 100.00%   100.00%        
Letter of Credit Percent of Collateral 100.00%   100.00%        
Terminated Interest Rate Swap Liability $ 31   $ 31        
Fuel derivatives              
Derivative [Line Items]              
Cash collateral held (from) by CP 32   32        
Interest rate swap              
Derivative [Line Items]              
Interest Rate Derivative           12 12
Interest rate derivatives              
Derivative [Line Items]              
Cash collateral held (from) by CP 0   0        
Discontinuation of Interest Rate Cash Flow Hedge Balance in AOCI   $ 32          
Not Designated as Hedging Instrument | Fuel derivatives              
Derivative [Line Items]              
Loss on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring 14   16        
Not Designated as Hedging Instrument | Interest rate derivatives              
Derivative [Line Items]              
Discontinuation of Interest Rate Cash Flow Hedge Balance in AOCI 14   31        
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments $ 5   $ 29