Marketable Securities (Details)							3 Months Ended				9 Months Ended		12 Months Ended
	Jul. 02, 2021 USD ($) shares	Jun. 25, 2021 USD ($) shares	Nov. 30, 2020 USD ($) shares	Dec. 02, 2019 USD ($) shares	Sep. 26, 2018 shares	Jun. 28, 2013 USD ($) shares	Sep. 30, 2022 USD ($) contract shares	Jun. 30, 2022 shares	Dec. 31, 2021 USD ($) shares	Sep. 30, 2021 USD ($) shares	Sep. 30, 2022 USD ($) contract shares	Sep. 30, 2021 USD ($) shares	Dec. 31, 2020 shares	Nov. 30, 2019 USD ($)
Marketable Securities
Shares settled (in shares) | 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) | shares	5,278,011		250,742	93,562
Investment owned, balance, contracts | contract							0				0
Gross proceeds from sale of marketable securities											$ 437,820,000	$ 495,703,000
Gain (loss) on marketable securities											(7,470,000)	16,472,000
Unrealized gain (loss) on marketable securities											(80,413,000)	$ 41,485,000
Marketable securities							$ 1,031,000		$ 524,569,000		1,031,000
Cantor
Marketable Securities
Securities loaned, fair value									182,000,000
Common Stock
Marketable Securities
Amount recognized in connection with the earn-out including other income (loss) (in shares) | shares													992,247
Common Stock | Nasdaq
Marketable Securities
Marketable securities		$ 1,093,900,000							$ 524,600,000
Shares received from transaction (in shares) | shares								2,497,831	2,780,180	2,780,180
Number of shares sold in transaction (in shares) | shares										3,458,515		3,709,257
Gross proceeds from sale of marketable securities										$ 460,300,000	437,800,000	$ 495,700,000
Gain (loss) on marketable securities										11,800,000	(7,500,000)	$ 16,500,000
Unrealized gain (loss) on marketable securities							$ 58,100,000			$ 41,500,000	$ (80,100,000)
Common Stock | Nasdaq | Nasdaq
Marketable Securities
Number of shares pledged to be repurchased (in shares) | shares							0		866,791		0
Amount received from the value of shares pledged									$ 140,000,000
Common Stock | Nasdaq Earn-Out
Marketable Securities
Shares settled (in shares) | 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) | shares						6,945,729
Minimum | Common Stock
Marketable Securities
Gross revenue on earn-out receivable per year under common stock transaction						$ 25,000,000
Nasdaq Omx | Maximum
Marketable Securities
Shares received from transaction (in shares) | shares					992,247
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) | 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