Compensation - Summary of the BGC Holdings and Newmark Holdings LPUs (Detail) - LPUs [Member] - shares	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding	102,407,000	79,729,000	58,323,000	50,835,000
Newmark [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding	14,607,000	22,113,000	26,510,000
Regular Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding	70,831,000
Regular Units [Member] | Newmark [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding	10,418,000
Preferred Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding	31,576,000
Preferred Units [Member] | Newmark [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding	4,189,000