Derivatives - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015	Jul. 29, 2011
Option Indexed to Issuer's Equity [Line Items]
Replacement cost of contracts in a gain position	$ 1,400,000
Adjustments to paid-in capital	11,400,000
GFI Group Inc. [Member]
Option Indexed to Issuer's Equity [Line Items]
Maximum exposure to economic loss	5,000,000
Capped Call [Member]
Option Indexed to Issuer's Equity [Line Items]
Adjustments to paid-in capital, net of taxes and issuance costs	9,900,000
4.50% Convertible Notes [Member]
Option Indexed to Issuer's Equity [Line Items]
Principal amount of debt component	$ 160,000,000	$ 160,000,000	$ 160,000,000
Stated interest rate	4.50%	4.50%	4.50%
4.50% Convertible Notes [Member] | Capped Call [Member]
Option Indexed to Issuer's Equity [Line Items]
Adjustments to paid-in capital	$ 11,400,000
Adjustments to paid-in capital, net of taxes and issuance costs	9,900,000
4.50% Convertible Notes [Member] | Embedded Conversion Feature [Member]
Option Indexed to Issuer's Equity [Line Items]
Adjustments to paid-in capital	19,000,000
Adjustments to paid-in capital, net of taxes and issuance costs	16,100,000
Short [Member] | Futures [Member]
Option Indexed to Issuer's Equity [Line Items]
Notional amounts of derivative contracts	5,800,000,000
Long [Member] | Futures [Member]
Option Indexed to Issuer's Equity [Line Items]
Notional amounts of derivative contracts	5,800,000,000
Not Designated as Hedging Instrument [Member]
Option Indexed to Issuer's Equity [Line Items]
Notional amounts of derivative contracts	$ 12,000,000,000	$ 10,900,000,000