Securities Owned and Securities Sold, but not yet Purchased (and Fair Value Measurement) - Ranges and Weighted Averages of Significant Unobservable Inputs Used to Determine Enterprise Value of Tradeworx (Detail) - Preferred Stock [Member] - Level 3 [Member]	Apr. 30, 2017
Minimum [Member] | Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average cost of capital ("WACC")	11.50%
Maximum [Member] | Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average cost of capital ("WACC")	14.50%
Weighted Average [Member] | Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average cost of capital ("WACC")	12.60%
Weighted Average [Member] | Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Revenue multiple	0.9