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	Sep. 30, 2017	Dec. 31, 2016
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned Total	$ 68,412	$ 58,907
Securities Sold, but not yet Purchased Total		1,248
Trading Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned Total	410	1,556
Securities Sold, but not yet Purchased Total		1,248
Available-for-sale Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned Total	$ 68,002	$ 57,351