Condensed Consolidated Statements Of Changes In Equity (USD $) In Thousands	Class A Common Stock [Member]	Class B Common Stock [Member]	Additional Paid-in Capital [Member]	Contingent Class A Common Stock [Member]	Treasury Stock [Member]	Retained Earnings (Deficit) [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interest in Subsidiaries [Member]	Total
Balance at Dec. 31, 2009	$ 707	$ 264	$ 292,881		$ (89,756)	$ (2,171)	$ (36)	$ 132,189	$ 334,078
Comprehensive income:
Consolidated net income						21,162		24,210	45,372
Other comprehensive income (loss), net of tax
Change in cumulative translation adjustment							(709)	(440)	(1,149)
Unrealized loss on securities available for sale							(232)	(105)	(337)
Comprehensive income (loss)						21,162	(941)	23,665	43,886
Stock-based compensation	8		7,724						7,732
Grant of exchangeability to limited partnership units								28,721	28,721
Capital contribution by founding/working partners to cover 2009 Charity Day			7,403						7,403
Dividends to stockholders						(42,606)			(42,606)
Earnings distributions to limited partnership interests								(45,192)	(45,192)
Issuance of Class A common stock upon exchange of founding/working partner units	52		10,644					5,627	16,323
Cantor purchase of Cantor units from BGC Holdings upon redemption of founding/working partner units								8,031	8,031
Cantor exchange of Cantor units for Class A common stock	37		6,144					(6,181)
Re-allocation of equity due to additional investment by founding/working partners								(21,681)	(21,681)
Proceeds from exercise of stock options, net of tax			463						463
Redemption of founding/working partner units								(10,292)	(10,292)
Repurchase of Class A common stock					(19,871)				(19,871)
Issuance of Class A common stock (net of costs) upon exchange of limited partnership units	45		26,439					(26,255)	229
Issuance of Class A common stock (net of costs), shares	26		15,134						15,160
Issuance of contingent Class A common stock and limited partnership units for acquisitions				3,171				3,566	6,737
Conversion of Class B common stock to Class A common stock	6	(6)
Other			(5)			(1)		2,741	2,735
Balance at Dec. 31, 2010	881	258	366,827	3,171	(109,627)	(23,616)	(977)	94,939	331,856
Comprehensive income:
Consolidated net income						18,330		16,257	34,587
Other comprehensive income (loss), net of tax
Change in cumulative translation adjustment							2,969	1,038	4,007
Unrealized loss on securities available for sale							(1,714)	(595)	(2,309)
Comprehensive income (loss)						18,330	1,255	16,700	36,285
Stock-based compensation	15		3,271						3,286
Grant of exchangeability to limited partnership units								34,043	34,043
Dividends to stockholders						(34,136)			(34,136)
Earnings distributions to limited partnership interests								(35,482)	(35,482)
Issuance of Class A common stock upon exchange of Cantor units	90		8,317					(8,407)
Issuance of Class B common stock upon exchange of Cantor units		90	8,317					(8,407)
Issuance of Class A common stock upon exchange of founding/working partner units	6		1,568					133	1,707
Re-allocation of equity due to additional investment by founding/working partners								(4,322)	(4,322)
Proceeds from exercise of stock options, net of tax	16		8,179						8,195
Repurchase of Class A common stock					(126)				(126)
Issuance of Class A common stock (net of costs) upon exchange of limited partnership units	13		10,589					(10,507)	95
Issuance of Class A common stock (net of costs), shares	16		13,574						13,590
Redemption of founding/working partner units			(226)					(9)	(235)
Excess tax benefit on exercise/delivery of equity awards			2,760						2,760
Other	2		(9)					(324)	(331)
Balance at Jun. 30, 2011	$ 1,039	$ 348	$ 423,167	$ 3,171	$ (109,753)	$ (39,422)	$ 278	$ 78,357	$ 357,185