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Investments - Additional Information (Detail)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2020
USD ($)
security
Sep. 30, 2019
USD ($)
Sep. 30, 2020
USD ($)
security
Sep. 30, 2019
USD ($)
Dec. 31, 2019
USD ($)
Investments, Debt and Equity Securities [Abstract]          
Fair value of investments, assets managed on behalf of the trade capital providers $ 150.2   $ 150.2   $ 158.6
Number of securities in an unrealized loss position, total | security 5,011   5,011    
Number of securities in an unrealized loss position for less than one year | security 1,411   1,411    
Number of securities in an unrealized loss position for a period of one year or greater | security 174   174    
Impairment losses $ 0.0 $ 5.0 $ 0.0 $ 11.8  
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]          
Credit losses on fixed maturities 10.5 $ 0.0 43.0 $ 0.0  
Gain (Loss) on Investments          
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]          
Credit losses on fixed maturities $ 10.5   $ 43.0