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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Fully diluted earnings per share
Net income available to common stockholders	$ 9,347	$ 7,601	$ 23,402	$ 15,609
Allocation of net income to limited partnership interests in BGC Holdings, net of tax	3,909	3,504	10,595	7,052
Dividend equivalent expense on RSUs, net of tax				2
Net income for fully diluted shares	$ 13,256	$ 11,105	$ 33,997	$ 22,663
Weighted-average shares:
Common stock outstanding	244,862	220,770	233,504	220,689
Limited partnership interests in BGC Holdings	120,122	104,083	117,359	102,071
RSUs (Treasury stock method)	944	685	944	762
Other	846	1,048	900	778
Fully diluted weighted-average shares of common stock outstanding	366,774	326,586	352,707	324,300
Fully diluted earnings per share	$ 0.04	$ 0.03	$ 0.10	$ 0.07