Condensed Consolidated Statements of Changes in Equity (Parenthetical) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Equity-based compensation (in shares)	213,459	114,967
Grant of exchangeability and redemption of limited partnership interests, issuance of shares (in shares)	8,446,093	1,746,526	49,033,008	5,820,715
Repurchase of Class A common stock (in shares)			41,940,000
Forfeiture of Class A Common Stock (in shares)	56,000	0	140,000	0
Cantor purchase of Cantor units from BGC Holdings upon redemption of FPUs (in shares)			2,768,060
Dividends declared per share of common stock (in dollars per share)	$ 0.01	$ 0.01	$ 0.03	$ 0.16
Dividends declared and paid per share of common stock (in dollars per share)	$ 0.01	$ 0.01	$ 0.03	$ 0.16
Partnership Units
Redemption of FPUs (in shares)	56,730	26,594
Class A Common Stock
Equity-based compensation (in shares)			1,940,975	914,641
Grant of exchangeability and redemption of limited partnership interests, issuance of shares (in shares)	8,446,000	1,747,000	49,033,000	5,821,000
Issuance of Class A common stock (net of costs) (in shares)	100,128	91,034	366,043	339,617
Repurchase of Class A common stock (in shares)	24,432,718	2,259	41,939,724	2,259
Forfeiture of Class A Common Stock (in shares)	56,423	0	140,188	0
Issuance of Class A common stock and RSUs for acquisitions (in shares)	343,000	42,000	1,130,000	327,000
Class A Common Stock | Partnership Units
Redemption of FPUs (in shares)			718,434	27,844
Class A Common Stock | Smith Mack
Issuance of Class A common stock and RSUs for acquisitions (in shares)	342,162	41,593	1,129,630	327,028