Limited Partnership Interests in BGC Holdings and Newmark Holdings - Additional Information (Detail) shares in Millions		6 Months Ended
	Nov. 23, 2018 shares	Jun. 30, 2021 partnership installment shares
Related Party Transaction [Line Items]
Number of operating partnerships | partnership		2
Number Of Installments | installment		4
LPUs
Related Party Transaction [Line Items]
Limited partnership interests exchange ratio		0.9403
Newmark Holdings
Related Party Transaction [Line Items]
Calculation of contribution ratio, denominator		2.2
Limited partnership interest, conversion ratio		1
Percentage of preferred partnership units awarded		0.6875%
Percentage of preferred partnership units awarded per calendar year		2.75%
BGC Holdings
Related Party Transaction [Line Items]
Limited partnership interest, conversion ratio		1
Percentage of preferred partnership units awarded		0.6875%
Percentage of preferred partnership units awarded per calendar year		2.75%
Cantor Rights to Purchase Exchangeable Units
Related Party Transaction [Line Items]
Right to exchange from Class A to Class B common stock (up to) (in shares) | shares	10.3	23.6