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Related Party Transactions - Other Related Party Transactions (Details) - USD ($)		3 Months Ended	6 Months Ended
	Nov. 30, 2018	Jun. 30, 2020	Jun. 30, 2020	Jun. 30, 2019	May 15, 2020	Dec. 31, 2019
Related Party Transaction
Payables to related parties		$ 3,880,000	$ 3,880,000			$ 38,090,000
Receivables from related parties		1,606,000	1,606,000			$ 0
Operating lease payments			23,900,000	$ 21,800,000
Cantor Credit Agreement \| Cantor
Related Party Transaction
Maximum revolving credit	$ 250,000,000
Basis spread on variable rate (in percentage)	1.00%
BGC Holdings, L.P. \| Affiliated Entity
Related Party Transaction
Operating lease payments		$ 200,000	$ 200,000
BGC Holdings, L.P. \| Affiliated Entity
Related Party Transaction
Receivables from related parties					$ 1,100,000