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, 2017	Mar. 31, 2016
Fully diluted (loss) earnings per share
Net income (loss) available to common stockholders	$ 18,215	$ 12,505
Allocations of net income (loss) to limited partnership interests in BGC Holdings, net of tax	9,395	6,424
Interest expense on convertible notes, net of tax		1,523
Net income (loss) for fully diluted shares	$ 27,610	$ 20,452
Weighted-average shares:
Common stock outstanding	283,399	273,780
Limited partnership interests in BGC Holdings	159,271	142,695
Convertible notes		16,260
RSUs (Treasury stock method)	677	858
Other	1,479	1,262
Fully diluted weighted-average shares of common stock outstanding	444,826	434,855
Fully diluted earnings (loss) per share	$ 0.06	$ 0.05