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Related Party Transactions - Other Related Party Transactions (Details) - USD ($)		3 Months Ended	9 Months Ended
	Nov. 30, 2018	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	May 15, 2020	Dec. 31, 2019
Related Party Transaction
Receivables from related parties		$ 652,000	$ 652,000			$ 0
Payables to related parties		5,185,000	5,185,000			$ 38,090,000
Operating lease payments			36,400,000	$ 32,900,000
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	$ 500,000
BGC Holdings, L.P. \| Affiliated Entity
Related Party Transaction
Receivables from related parties					$ 1,100,000
Lessor, Operating Lease, Term of Contract					1 year