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Investments in debt and equity securities - Additional Information (Detail) - Investments in mutual funds and common stock - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Schedule Of Trading Securities And Other Trading Assets [Line Items]    
Pre-tax Net Gain (Loss) recognized in Income Statement $ 2,776 $ 1,597
Pre-tax Realized Gain (Loss) on Securities Arising During Period 586 4,101
Net Increase (Decrease) in Unrealized Gain on Equity Securities due to changes in Fair Value $ 2,190 $ 2,504