Compensation - Summary of the BGC Holdings and Newmark Holdings LPUs held by BGC Employees (Detail) - LPUs - shares	Sep. 30, 2021	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Balance outstanding (in shares)	122,559,000	137,652,000
Newmark
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Balance outstanding (in shares)	11,123,000	13,202,000
Regular Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Balance outstanding (in shares)	85,168,000
Regular Units | Newmark
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Balance outstanding (in shares)	8,361,000
Preferred Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Balance outstanding (in shares)	37,391,000
Preferred Units | Newmark
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Balance outstanding (in shares)	2,762,000