Limited Partnership Interests in BGC Holdings - Additional Information (Detail)	3 Months Ended				6 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2017 Partnership	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%	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%				2.75%