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Investments in debt and equity securities - Additional Information (Detail) - Investments in mutual funds and common stock - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Schedule Of Trading Securities And Other Trading Assets [Line Items]    
Pre-tax Net Gain (Loss) recognized in Income Statement $ (3,407) $ 1,893
Pre-tax Realized Gain (Loss) on Securities Arising During Period 293 170
Net Increase Decrease In Unrealized Gains On Equity Securities $ (3,700) $ 1,723