Securities Owned and Securities Sold, but not yet Purchased (and Fair Value Measurement) - Schedule of Unrealized Gains, Losses and Fair Value of AFS Securities (Detail) - Available-for-sale Securities [Member] - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 57,615
Gross unrealized gains in other comprehensive income	0	$ 0
Gross unrealized losses in other comprehensive income	(264)
Fair value	$ 57,351