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Securities Owned/Sold but Not Yet Purchased - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Fair Value Disclosures [Abstract]        
Proceeds from sale and maturity of available-for-sale securities $ 4,000 $ 25,065 $ 64,498 $ 46,065
Recognized gross realized losses $ 23 $ 236 $ 739 $ 412