Derivatives - Additional Information (Detail) - USD ($)			3 Months Ended	12 Months Ended
	Jul. 15, 2016	Jul. 13, 2016	Mar. 31, 2017	Dec. 31, 2016	Jul. 29, 2011
Option Indexed to Issuer's Equity [Line Items]
Replacement cost of contracts in a gain position			$ 4,200,000
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%
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		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
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
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
Short [Member] | Futures [Member]
Option Indexed to Issuer's Equity [Line Items]
Notional amounts of derivative contracts			15,600,000,000
Long [Member] | Futures [Member]
Option Indexed to Issuer's Equity [Line Items]
Notional amounts of derivative contracts			15,600,000,000
GFI Group Inc. [Member]
Option Indexed to Issuer's Equity [Line Items]
Maximum exposure to economic loss			2,700,000
Not Designated as Hedging Instrument [Member]
Option Indexed to Issuer's Equity [Line Items]
Notional amounts of derivative contracts			$ 16,900,000,000	$ 12,100,000,000