Securities Owned/Sold but Not Yet Purchased - Schedule of Securities Owned/Sold But Not Yet Purchased (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned Total	$ 9,095	$ 8,873
Securities Sold, but not yet Purchased Total	543	1,698
Trading Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned Total	9,095	8,873
Securities Sold, but not yet Purchased Total	$ 543	$ 1,698