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Investments - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]          
Deposit Assets $ 261,000,000.0   $ 261,000,000.0    
Securities Sold under Agreements to Repurchase 0 $ 0 0 $ 0 $ 0
Open reverse repurchase commitments 400,000,000.0 0 400,000,000.0 0 0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment     $ 0 $ 0  
Investment Portfolio Duration     2 years 10 months 24 days 2 years 7 months 6 days  
Trading securities 0 0 $ 0 $ 0 0
Trading Securities, Gain (Loss) 0 0 0 0  
Derivative Assets (Liabilities), at Fair Value, Net $ 0 $ 0 $ 0 $ 0 $ 0