Marketable Securities (Details) - USD ($)						3 Months Ended			9 Months Ended		36 Months Ended
	Jul. 02, 2021	Jun. 25, 2021	Nov. 30, 2020	Dec. 02, 2019	Jun. 28, 2013	Sep. 30, 2023	Sep. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2020	Dec. 31, 2022	Jun. 30, 2021	Nov. 30, 2019
Marketable Securities
Shares settled (in shares)	944,329		741,505	898,685
Investment owned shares settled fair value	$ 166,000,000		$ 93,500,000	$ 93,500,000
Shares owned (in shares)	5,278,011		250,742	93,562
Gross proceeds from sale of marketable securities									$ 0	$ 437,820,000
Realized gain (loss) on marketable securities									0	(7,470,000)
Unrealized gain (loss) on marketable securities									(548,000)	(80,413,000)
Marketable securities						$ 239,000			$ 239,000			$ 788,000
Common Stock
Marketable Securities
Amount recognized in connection with the earn-out including other income (loss) (in shares)											992,247
Common Stock | Nasdaq
Marketable Securities
Marketable securities													$ 1,093,900,000
Number of shares sold in transaction (in shares)								2,497,831	0
Gross proceeds from sale of marketable securities										437,800,000
Realized gain (loss) on marketable securities									$ 0	(7,500,000)
Unrealized gain (loss) on marketable securities									0	(80,100,000)
Common Stock | Securites In Public Entities
Marketable Securities
Realized gain (loss) on marketable securities						0	$ 0		0	0
Unrealized gain (loss) on marketable securities						$ 100,000	$ 0		$ 600,000	$ 0
Common Stock | Nasdaq Earn-Out
Marketable Securities
Shares settled (in shares)	944,329	6,222,340
Marketable securities	$ 166,000,000		$ 121,900,000
Marketable securities														$ 98,600,000
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
BGC Partners Inc | Common Stock
Marketable Securities
Period for earn-out receivable under common stock transaction					7 years
BGC Partners Inc | Nasdaq Omx
Marketable Securities
Period for earn-out receivable under common stock transaction					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