Marketable Securities (Details) - USD ($)		12 Months Ended
	Jun. 28, 2013	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Nov. 30, 2019
Marketable Securities [Line Items]
Gross proceeds from sale of marketable securities		$ 32,606,000	$ 95,878,000	$ 18,710,000
Gain (loss) on marketable securities		4,056,000	3,256,000	0
Unrealized (gains) loss on marketable securities		11,303,000	(1,193,000)	0
Marketable securities		$ 36,795,000	$ 48,942,000		$ 98,600,000
Nasdaq Omx
Marketable Securities [Line Items]
Amount recognized in connection with the earn-out including other income (loss) (in shares)		992,247	992,247
Gain recognized on earn out receivable under common stock transaction		$ 98,600,000	$ 85,100,000	$ 77,000,000.0
Number of shares sold in transaction (in shares)		350,000	1,142,247
Remaining number of earn-out shares received under common stock transaction (in shares)		343,562
Nasdaq Omx | BGC Partners Inc
Marketable Securities [Line Items]
Period for earn-out receivable under common stock transaction	15 years	8 years
Maximum | Nasdaq Omx
Marketable Securities [Line Items]
Remaining earn-out receivable under common stock transaction (in shares)		7,937,976
Maximum | Nasdaq Omx | BGC Partners Inc
Marketable Securities [Line Items]
Earn-out shares receivable under common stock transaction (in shares)	14,883,705
Minimum | Nasdaq Omx
Marketable Securities [Line Items]
Gross revenue on earn-out receivable per year under common stock transaction	$ 25,000,000.0
Minimum | Nasdaq Omx | BGC Partners Inc
Marketable Securities [Line Items]
Gross revenue on earn-out receivable per year under common stock transaction	$ 25,000,000.0