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Available-for-Sale Debt Securities - Additional Information (Details)
12 Months Ended
Dec. 31, 2020
USD ($)
security
Dec. 31, 2019
USD ($)
security
Debt Securities, Available-for-sale [Abstract]    
Accrued interest receivable $ 9,000,000 $ 37,000,000
Securities in unrealized loss positions, number of positions (securities) | security 75 305
Impairment loss recognized $ 0 $ 0