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Investments in debt and equity securities - Additional Information (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Jan. 01, 2018
Investments in mutual funds and common stock      
Schedule Of Trading Securities And Other Trading Assets [Line Items]      
Pre-tax Net Gain (Loss) recognized in Income Statement $ 1,597    
Pre-tax Realized Gain (Loss) on Securities Arising During Period 4,101 $ (362)  
Net Decrease in Unrealized Gain on Equity Securities due to changes in Fair Value $ 2,504    
Gain on Sale Of Equity Securities After Tax   $ (221)  
Investment securities      
Schedule Of Trading Securities And Other Trading Assets [Line Items]      
Cumulative Effect of New Accounting Principle in Period of Adoption     $ (5,662)