Condensed Consolidated Statements of Changes in Equity (Unaudited) (Parenthetical)	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Repurchase of Class A common stock, shares	44,013
Partnership Units
Redemption of founding/working partner units	2,027,762	236,741
Cantor purchase of Cantor units from BGC Holdings upon redemption of founding/working partner units	920,729
Class A Common Stock
Equity-based compensation, shares	1,172,546	1,937,093
Grant of exchangeability and redemption of limited partnership interests, shares	12,297,169	12,259,184
Issuance of Class A common stock (net of costs), shares	2,969,927	3,829,176
Issuance of common stock upon exchange of Cantor units, shares		9,000,000
Issuance of Class A common stock upon purchase of notes receivable, shares	453,172
Repurchase of Class A common stock, shares	44,013	60,929
Contingent Class A common stock for acquisitions, shares	924,241	4,716,848
Class B Common Stock
Issuance of common stock upon exchange of Cantor units, shares		9,000,000