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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, 2017
Dec. 31, 2016
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities Owned Total $ 57,611 $ 58,907
Securities Sold, but not yet Purchased Total 27 1,248
Trading Securities [Member]    
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities Owned Total 264 1,556
Securities Sold, but not yet Purchased Total 27 1,248
Available-for-sale Securities [Member]    
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]    
Securities Owned Total $ 57,347 $ 57,351