Compensation - Compensation Expense Related to Issuance of BGC or Newmark Class A Common Stock and Grants of Exchangeability on BGC Holdings and Newmark Holdings LPUs to BGC Employees (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock and exchangeability expenses	$ 100,948	$ 150,147	$ 176,463