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Investments - Additional Information (Detail)
$ in Millions
Dec. 31, 2018
USD ($)
Security
Dec. 31, 2017
USD ($)
Security
Investments Debt And Equity Securities [Abstract]    
Available-for-sale, investment securities held with maturity dates more than one year and less than two years | $ $ 31.8 $ 48.7
Number of securities in unrealized loss position | Security 57 54