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Securities Owned/Sold but Not Yet Purchased - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Fair Value Disclosures [Abstract]      
Proceeds from sale and maturity of available-for-sale securities $ 64,498 $ 91,095 $ 6,002
Recognized gross realized losses $ 739 $ 1,132