Earnings Per Share - Calculation of Fully Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Fully diluted (loss) earnings per share
Net income (loss) available to common stockholders	$ 21,768	$ 14,467	$ 39,983	$ 26,972
Allocations of net income (loss) to limited partnership interests in BGC Holdings, net of tax	11,326	7,464	20,721	13,888
Interest expense on convertible notes, net of tax		1,521		3,044
Net income (loss) for fully diluted shares	$ 33,094	$ 23,452	$ 60,704	$ 43,904
Weighted-average shares:
Common stock outstanding	286,840	275,997	285,129	274,895
Limited partnership interests in BGC Holdings	163,327	143,229	161,311	142,963
Convertible notes		16,260		16,260
RSUs (Treasury stock method)	409	376	525	517
Other	1,281	1,395	1,382	1,328
Fully diluted weighted-average shares of common stock outstanding	451,857	437,257	448,347	435,963
Fully diluted earnings (loss) per share	$ 0.07	$ 0.05	$ 0.14	$ 0.10