Marketable Securities (Details) - USD ($)		3 Months Ended			9 Months Ended		12 Months Ended
	Jun. 28, 2013	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Marketable Securities
Gross proceeds from sale of marketable securities					$ 34,738,000	$ 32,606,000
Gain (loss) on marketable securities					(2,204,000)	4,056,000
Unrealized (gains) loss on marketable securities					0	$ 4,445,000
Common Stock
Marketable Securities
Amount recognized in connection with the earn-out including other income (loss) (in shares)							992,247
Number of shares sold in transaction (in shares)			343,562	100,000		350,000
Gross proceeds from sale of marketable securities			$ 34,700,000	$ 10,400,000		$ 32,600,000
Gain (loss) on marketable securities			$ 2,200,000	2,200,000		4,100,000
Unrealized (gains) loss on marketable securities				$ (700,000)		$ 4,400,000
Marketable securities		$ 121,900,000			$ 121,900,000		$ 36,800,000
Common Stock | Nasdaq Earn-Out
Marketable Securities
Gain (loss) on marketable securities		$ 121,900,000
Maximum | Common Stock
Marketable Securities
Remaining earn-out receivable under common stock transaction (in shares)	6,945,729
Minimum | Common Stock
Marketable Securities
Gross revenue on earn-out receivable per year under common stock transaction	$ 25,000,000.0
BGC Partners Inc | Common Stock
Marketable Securities
Period for earn-out receivable under common stock transaction (years)					7 years
BGC Partners Inc | Nasdaq Omx
Marketable Securities
Period for earn-out receivable under common stock transaction (years)	15 years
BGC Partners Inc | Nasdaq Omx | Maximum
Marketable Securities
Earn-out shares receivable under common stock transaction (in shares)	14,883,705
BGC Partners Inc | Nasdaq Omx | Minimum
Marketable Securities
Gross revenue on earn-out receivable per year under common stock transaction	$ 25,000,000.0