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Financial Derivative Instruments Narrative (Details)
$ in Millions
3 Months Ended
Mar. 31, 2020
USD ($)
Mar. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Agreements
Derivative [Line Items]      
Premiums paid for fuel derivative contracts $ 24 $ 28  
Percentage of actual consumption hedged 70.00%    
Current Unrealized Net Losses in OCI $ 67    
Maximum sum of derivatives of counterparty to be included in other (less than $8 million) $ 8    
Cash Collateral Percent Of Fair Value Fuel Derivatives Contracts 100.00%    
Letter of Credit Percent of Collateral 100.00%    
Fuel derivatives      
Derivative [Line Items]      
Cash collateral held (from) by CP $ 20    
Interest rate swap      
Derivative [Line Items]      
Interest Rate Derivative   12 12
Interest rate derivatives      
Derivative [Line Items]      
Cash collateral held (from) by CP 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 2    
Not Designated as Hedging Instrument | Interest rate derivatives      
Derivative [Line Items]      
Discontinuation of Interest Rate Cash Flow Hedge Balance in AOCI 17    
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments $ 24    
No Longer Probable Within Scheduled Delivery Range | Interest rate swap      
Derivative [Line Items]      
Interest Rate Derivative | Agreements     9