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Fair Value of Financial Assets and Liabilities - Interest Rate Derivatives (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Cash Flow Hedges | Designated as Hedging Instrument    
Derivative [Line Items]    
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net $ 1 $ 1
Interest Rate Swap    
Derivative [Line Items]    
Amount of accumulated other comprehensive gains (losses) related to interest rate derivatives expected to be reclassified into earnings within the next twelve months 1  
Interest Rate | Cash Flow Hedges | Designated as Hedging Instrument    
Derivative [Line Items]    
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net [1] $ 1 $ 1
[1] Amounts are recorded to interest charges.