Securities Owned/Sold but Not Yet Purchased - Schedule of Securities Owned/Sold But Not Yet Purchased (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned Total	$ 4,426	$ 66,294
Securities Sold, but not yet Purchased Total	2,018	950
Trading Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned Total	4,426	1,691
Securities Sold, but not yet Purchased Total	$ 2,018	950
Available-for-sale Debt Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned Total		$ 64,603