Earnings Per Share - Calculation of Fully Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Fully diluted earnings per share
Net income available to common stockholders	$ 13,659	$ 14,055
Allocation of net income to limited partnership interests in BGC Holdings, net of tax	7,020	6,686
Interest expense on convertible notes, net of tax	1,524
Net income for fully diluted shares	$ 22,203	$ 20,741
Weighted-average shares:
Common stock outstanding	273,780	222,019
Limited partnership interests in BGC Holdings	139,825	114,564
Convertible notes	16,260
RSUs (Treasury stock method)	858	944
Other	4,132	957
Fully diluted weighted-average shares of common stock outstanding	434,855	338,484
Fully diluted earnings per share	$ 0.05	$ 0.06