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Securities Owned and Securities Sold, but not yet Purchased (and Fair Value Measurement) - Schedule of Unrealized Gains, Losses and Fair Value of AFS Debt Securities (Detail) - Available-for-sale Debt Securities [Member] - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 4,022 $ 65,237
Gross unrealized gains in other comprehensive income 0 0
Gross unrealized losses in other comprehensive income (22) (634)
Fair value $ 4,000 $ 64,603