Earnings Per Share - Calculation of Fully Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Fully diluted earnings per share:
Net income available to common stockholders	$ 7,211		$ 25,326		$ 22,820		$ 66,790
Allocation of net income to limited partnership interests in BGC Holdings, net of tax	3,538		10,601		10,590		49,962
Interest expense on convertible notes, net of tax			4,673				12,161
Dividend equivalent expense on RSUs, net of tax					2		3
Net income for fully diluted shares	$ 10,749		$ 40,600		$ 33,412		$ 128,916
Weighted-average shares:
Common stock outstanding	220,388		219,174		220,588		184,924
Limited partnership interests in BGC Holdings	108,912		93,954		104,376		137,249
Convertible notes			39,855				39,767
RSUs (Treasury stock method)	806		925		759		286
Other	1,103		1,259		887		922
Fully diluted weighted-average shares of common stock outstanding	331,209	[1]	355,167	[1]	326,610	[1]	363,148	[1]
Fully diluted earnings per share	$ 0.03		$ 0.11		$ 0.10		$ 0.35

[1] (1) For the three months ended September 30, 2014 and 2013, respectively, approximately 44.2 million and 6.4 million potentially dilutive securities were not included in the computation of fully diluted EPS because their effect would have been anti-dilutive. Anti-dilutive securities for the three months ended September 30, 2014 included, on a weighted-average basis, 40.2 million shares underlying Convertible Notes and 4.0 million other securities or other contracts to issue shares of common stock.