Earnings Per Share - Additional Information (Detail) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Limited Partnership Unit [Member] | Contingent Class A Common Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount	15,800,000	1,500,000	1,100,000
BGC Holdings Partnership Unit [Member] | Contingent Class A Common Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount	0	15,600,000
Continuing Operations [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount	900,000	163,000,000.0	167,000,000.0
Continuing Operations [Member] | Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount	900,000	200,000	2,000,000.0
Continuing Operations [Member] | Limited Partnership Unit [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount		162,800,000	165,000,000.0
Discontinued Operations [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount		163,000,000.0	167,000,000.0
Discontinued Operations [Member] | Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount		200,000	2,000,000.0
Discontinued Operations [Member] | Limited Partnership Unit [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount		162,800,000	165,000,000.0