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Derivative Financial Instruments - Credit-Risk Related Contingent Features (Detail)
$ in Millions
Dec. 31, 2020
USD ($)
One-notch Downgrade [Member]  
Derivative [Line Items]  
Amount of additional collateral to be posted upon downgrade $ 0
Two-notch Downgrade [Member]  
Derivative [Line Items]  
Amount of additional collateral to be posted upon downgrade $ 121