Earnings Per Share - Additional Information (Detail) - shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)	143.5	187.4	98.2	2.9
Limited Partnership Unit
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)	137.6	184.7	98.1
Contingent Class A Common Stock | BGC Holdings Partnership Unit
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)			37.5	28.4
Contingent Class A Common Stock | Limited Partnership Unit
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)			37.5	28.4
Contingent Class A Common Stock | N Units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)			37.5	28.4
Restricted Stock Units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)		2.7	0.1
Restricted Stock Units | Contingent Class A Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)			37.5	28.4
Other Antidilutive Securities
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)	5.9