Condensed Consolidated Statements of Changes in Equity (Unaudited) (USD $) In Thousands, unless otherwise specified	Total	Class A Common Stock	Class B Common Stock	Additional Paid-in Capital	Contingent Class A Common Stock	Treasury Stock	Retained Earnings (Deficit)	Accumulated Other Comprehensive Loss	Noncontrolling Interest in Subsidiaries
Balance at Dec. 31, 2010	$ 331,856	$ 881	$ 258	$ 366,827	$ 3,171	$ (109,627)	$ (23,616)	$ (977)	$ 94,939
Comprehensive income:
Consolidated net income	38,360						20,137		18,223
Other comprehensive income (loss) net of tax
Change in cumulative translation adjustment	(3,979)							(3,471)	(508)
Unrealized gain (loss) on securities available for sale	671							696	(25)
Comprehensive income (loss)	35,052						20,137	(2,775)	17,690
Equity-based compensation	8,721	19		4,337					4,365
Dividends to common stockholders	(77,244)						(77,244)
Earnings distributions to limited partnership interests	(69,816)								(69,816)
Grant of exchangeability and redemption of limited partnership interests, issuance of shares	111,887	123		79,928					31,836
Issuance of Class A common stock (net of costs)	26,765	39		14,774					11,952
Issuance of Class A common stock upon exchange of Cantor units, 9,000,000 shares		90		8,317					(8,407)
Issuance of Class B common stock upon exchange of Cantor units, 9,000,000 shares			90	8,317					(8,407)
Redemption of founding/working partner units	(926)			(395)					(531)
Repurchase of Class A common stock	(392)					(243)			(149)
Capital contribution by founding/working partners with respect to Charity Day	8,176			5,130					3,046
Re-allocation of equity due to additional investment by founding/working partners	(6,121)								(6,121)
Purchase of capped call, net of tax	(9,911)			(6,219)					(3,692)
Equity component of convertible notes, net of tax	16,053			10,073					5,980
Acquisition of CantorCO2e, L.P.	(2,000)			(1,255)					(745)
Issuance of contingent Class A common stock for acquisitions	26,778			236	16,962				9,580
Newmark noncontrolling interest	14,384								14,384
Other	1,436			(701)			(3)		2,140
Balance at Dec. 31, 2011	414,698	1,152	348	489,369	20,133	(109,870)	(80,726)	(3,752)	98,044
Comprehensive income:
Consolidated net income	17,079						9,696		7,383
Other comprehensive income (loss) net of tax
Change in cumulative translation adjustment	(1,275)							(1,057)	(218)
Comprehensive income (loss)	15,804						9,696	(1,057)	7,165
Equity-based compensation	3,808	12		1,970					1,826
Dividends to common stockholders	(72,284)						(72,284)
Earnings distributions to limited partnership interests	(66,167)								(66,167)
Grant of exchangeability and redemption of limited partnership interests, issuance of shares	92,196	123		45,426					46,647
Issuance of Class A common stock (net of costs)	18,314	29		11,912					6,373
Issuance of Class A common stock upon purchase of notes receivable	3,055	5		1,991					1,059
Redemption of founding/working partner units	(10,846)			(7,064)					(3,782)
Repurchase of Class A common stock	(337)					(220)			(117)
Cantor purchase of Cantor units from BGC Holdings upon redemption of founding/working partner units, 920,729 units	2,732								2,732
Re-allocation of equity due to additional investment by founding/working partners	(144)								(144)
Issuance of contingent Class A common stock for acquisitions	725	9		4,949	(4,469)				236
Newmark noncontrolling interest	(4,382)			(828)	386				(3,940)
Other	(1,561)			(1,497)					(64)
Balance at Sep. 30, 2012	$ 395,611	$ 1,330	$ 348	$ 546,228	$ 16,050	$ (110,090)	$ (143,314)	$ (4,809)	$ 89,868