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Related Party Transactions - Other Related Party Transactions (Details) - USD ($)		12 Months Ended
	Nov. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	May 15, 2020
Related Party Transaction
Accounts receivable, related parties		$ 0	$ 0
Payables to related parties		4,392,000	38,090,000
Operating lease payments		49,000,000.0	$ 44,400,000
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		$ 800,000
BGC Holdings, L.P. \| Affiliated Entity
Related Party Transaction
Accounts receivable, related parties				$ 1,100,000
Lease, term of contract (in years)				1 year