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Securities Owned/Sold but Not Yet Purchased - Schedule of Securities Owned/Sold But Not Yet Purchased (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities Owned Total $ 3,376 $ 8,873
Securities Sold, but not yet Purchased Total 568 1,698
Trading Securities [Member]    
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities Owned Total 3,376 8,873
Securities Sold, but not yet Purchased Total $ 568 $ 1,698