Earnings Per Share - Calculation of Fully Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fully diluted (loss) earnings per share
Net income (loss) available to common stockholders	$ 81,529	$ 87,211	$ 171,274	$ 140,583
Allocations of net income (loss) to limited partnership interests in BGC Holdings, net of tax	45,966	44,693	94,727	72,147
Interest expense on convertible notes, net of tax		253		3,297
Net income (loss) for fully diluted shares	$ 127,495	$ 132,157	$ 266,001	$ 216,027
Weighted-average shares:
Common stock outstanding	288,308	278,601	286,200	276,144
Partnership units	167,287	147,222	163,325	144,341
Convertible notes		2,121		11,495
RSUs (Treasury stock method)	539	423	505	425
Other	1,207	1,394	1,318	2,308
Fully diluted weighted-average shares of common stock outstanding	457,341	429,761	451,348	434,713
Fully diluted earnings (loss) per share	$ 0.28	$ 0.31	$ 0.59	$ 0.50