Earnings Per Share - Schedule of Allocations of Earnings (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Abstract]
Net income (loss) available to common stockholders	$ 25,326	$ (451)	$ 66,790	$ 9,696
Allocation of income to limited partnership interests in BGC Holdings	10,686	85	161,712	13,556
Fully diluted earnings per share:
Net income (loss) available to common stockholders	25,326	(451)	66,790	9,696
Allocation of net income to limited partnership interests in BGC Holdings, net of tax	10,601		49,962	8,953
Interest expense on convertible notes, net of tax	4,673		12,161
Dividend equivalent expense on RSUs, net of tax			3	195
Net income (loss) for fully diluted shares	$ 40,600	$ (451)	$ 128,916	$ 18,844
Weighted-average shares:
Common stock outstanding	219,174	146,703	184,924	141,104
Limited partnership interests in BGC Holdings	93,954		137,249	132,246
Convertible notes	39,855		39,767
RSUs (Treasury stock method)	925		286	618
Other	1,259		922	1,191
Fully diluted weighted-average shares of common stock outstanding	355,167	146,703	363,148	275,159
Fully diluted earnings per share	$ 0.11	$ 0.00	$ 0.35	$ 0.07