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Securities Owned/Sold but Not Yet Purchased - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Fair Value Disclosures [Abstract]        
Proceeds from sale and maturity of available-for-sale securities $ 0 $ 19,003 $ 64,498 $ 65,067
Recognized gross realized losses $ 0 $ 277 $ 739 $ 687