Earnings Per Share and Weighted-Average Shares Outstanding - Calculation of Fully Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Fully diluted earnings per share
Net income available to common stockholders		$ 19,997	$ 36,711
Allocations of net income to limited partnership interests in BGC Holdings and Newmark Holdings, net of tax		10,289
Net income for fully diluted shares		$ 30,286
Weighted-average shares:
Basic weighted-average shares of common stock outstanding		155,694
Partnership units	[1]	90,222
Other		918
Fully diluted weighted-average shares of common stock outstanding		246,834
Fully diluted earnings (loss) per share		$ 0.12

[1] Partnership units collectively include founding/working partner units, limited partnership units, and Cantor units (see Note 2—Limited Partnership Interest in Newmark Holdings for more information).