Derivatives - Additional Information (Detail) - USD ($)			6 Months Ended	12 Months Ended
	Jul. 15, 2016	Jul. 13, 2016	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Jul. 29, 2011
Option Indexed to Issuer's Equity [Line Items]
Replacement cost of contracts in a gain position			$ 3,200,000
Class A Common Stock [Member]
Option Indexed to Issuer's Equity [Line Items]
Convertible Notes, shares				6,909
4.50% Convertible Notes [Member]
Option Indexed to Issuer's Equity [Line Items]
Principal amount of debt component						$ 160,000,000
Stated interest rate	4.50%	4.50%	4.50%		4.50%	4.50%
Principal amount of notes		$ 68,000
Repayments of convertible notes	$ 159,900,000
Capped call transactions expiration date		Jul. 13, 2016	Jul. 13, 2016
Senior Notes maturity date			Jul. 15, 2016
4.50% Convertible Notes [Member] | Class A Common Stock [Member]
Option Indexed to Issuer's Equity [Line Items]
Convertible Notes, shares		6,909
Short [Member] | Futures [Member]
Option Indexed to Issuer's Equity [Line Items]
Notional amounts of derivative contracts			$ 7,000,000,000
Long [Member] | Futures [Member]
Option Indexed to Issuer's Equity [Line Items]
Notional amounts of derivative contracts			7,200,000,000
GFI Group Inc. [Member]
Option Indexed to Issuer's Equity [Line Items]
Maximum exposure to economic loss			2,800,000
Not Designated as Hedging Instrument [Member]
Option Indexed to Issuer's Equity [Line Items]
Notional amounts of derivative contracts			$ 14,600,000,000	$ 12,100,000,000