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Securities Owned and Securities Sold, but not yet Purchased (and Fair Value Measurement) - Schedule of Securities Owned/Sold But Not Yet Purchased (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities Owned Total $ 4,585 $ 66,294
Securities Sold, but not yet Purchased Total   950
Trading Securities [Member]    
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities Owned Total 585 1,691
Securities Sold, but not yet Purchased Total   950
Available-for-sale Debt Securities [Member]    
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities Owned Total $ 4,000 $ 64,603