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Investments - Additional Information (Detail)
1 Months Ended 6 Months Ended
Mar. 31, 2020
USD ($)
Jun. 30, 2021
USD ($)
security
Dec. 31, 2020
USD ($)
Jun. 30, 2020
USD ($)
Schedule of Debt and Equity Securities [Line Items]        
Principal amount of bonds and certificates of deposit   $ 409,400,000    
Securities Exceeding 10% of Shareholders' Equity   0 $ 0 $ 0
Non-Income Producing Securities   0    
Total available-for-sale securities   44,741,600,000 42,029,400,000 39,426,900,000
Open reverse repurchase commitments   0 0 0
Securities Sold under Agreements to Repurchase   $ 0 0 0
Debt Securities, Available-for-sale, Unrealized Loss Position, Positions Downgraded | security   0    
Debt Securities, Available-for-sale, Allowance for Credit Loss, Not Previously Recorded   $ 0    
Debt Securities, Available-for-sale, Allowance for Credit Loss   0 $ 0 $ 0
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff   $ 0    
Percent Change in Investment Income   11.00%    
Percent Change in Book Yield   23.00%    
Proceeds from Debt, Net of Issuance Costs $ 1,000,000,000.0      
Foreign Government Debt        
Schedule of Debt and Equity Securities [Line Items]        
Total available-for-sale securities   $ 12,700,000