Limited Partnership Interests in BGC Holdings - Additional Information (Detail) - Partnership	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017
Noncontrolling Interest [Abstract]
Number of operating partnerships				2
Payout period for post-termination awards				Four equal yearly installments
Percentage to preferred units				0.6875%
Limited partnership interests Exchange ratio, subject to adjustment				1
Scenario, Forecast [Member]
Noncontrolling Interest [Abstract]
Percentage to preferred units	0.6875%	0.6875%	0.6875%		2.75%