Organization and Basis of Presentation (Details)								3 Months Ended			12 Months Ended		55 Months Ended
	Jul. 02, 2021 USD ($) shares	Jun. 28, 2021 USD ($) $ / shares shares	Jun. 25, 2021 USD ($) shares	Nov. 30, 2020 USD ($) shares	Dec. 02, 2019 USD ($) shares	Sep. 26, 2018 USD ($) tranche shares	Jun. 28, 2013 USD ($) shares	Mar. 31, 2022 USD ($) shares	Dec. 31, 2021 USD ($) shares	Sep. 30, 2021 shares	Dec. 31, 2021 USD ($) shares	Dec. 31, 2018 USD ($)	Mar. 31, 2022 USD ($) shares	Jul. 31, 2021 $ / shares	Nov. 30, 2019 USD ($)	Jun. 18, 2018 USD ($)
Description Of Business
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
Marketable securities | $								$ 0	$ 524,569,000		$ 524,569,000		$ 0
Common stock transactions, remaining (in shares)								0					0
2021 Equity Event
Description Of Business
Compensation expense | $		$ 428,600,000
Deferred Tax Assets, Net of Valuation Allowance | $		$ 101,000,000
Settlement share price (in usd per share) | $ / shares		$ 12.50												$ 13.01
2021 Equity Event | Class A Common Stock
Description Of Business
Limited partnership units conversion ratio		0.9403
Common Stock
Description Of Business
Amount recognized in connection with the earn-out including other income (loss) (in shares)											992,247
Common Stock | Nasdaq
Description Of Business
Shares received from transaction (in shares)								2,497,831	2,780,180	2,780,180
Marketable securities | $			$ 1,093,900,000						$ 524,600,000		$ 524,600,000
Common Stock | Nasdaq Earn-Out
Description Of Business
Marketable securities | $	$ 166,000,000			$ 121,900,000
Shares settled (in shares)	944,329		6,222,340
Marketable securities | $															$ 98,600,000
Newmark Holdings, L.P. | Employees | Limited Partnership Units
Description Of Business
Shares redeemed or exchanged (in shares)		8,300,000
Newmark Holdings, L.P. | Independent Contractors | Limited Partnership Units
Description Of Business
Shares redeemed or exchanged (in shares)		23,200,000
BGC Holdings, L.P. | Employees | Limited Partnership Units
Description Of Business
Shares redeemed or exchanged (in shares)		8,000,000
BGC Holdings, L.P. | Independent Contractors | Limited Partnership Units
Description Of Business
Shares redeemed or exchanged (in shares)		17,400,000
Nasdaq Omx | Maximum
Description Of Business
Shares received from transaction (in shares)						992,247
Nasdaq Omx | eSpeed
Description Of Business
Purchase consideration paid in cash | $							$ 750,000,000
Period for expected payment under Common stock transaction							15 years
Amount recognized in connection with the earn-out including other income (loss) (in shares)													10,200,000
Shares sold under common stock transactions (in shares)													7,600,000
Common stock transactions, realized gain and dividend income | $													$ 1,474,200,000
Nasdaq Omx | eSpeed | Maximum | BGC Partners Inc
Description Of Business
Expected payment of shares under common stock transaction (in shares)							14,883,705
Nasdaq Omx | eSpeed | Minimum | BGC Partners Inc
Description Of Business
Gross revenue on expected payment per year under common stock transaction | $							$ 25,000,000
RBC | Newmark OpCo
Description Of Business
Exchangeable preferred limited partnership units issued (in shares) | $						$ 150,000,000										$ 175,000,000
Proceeds from issuance of EPUs | $												$ 266,100,000
Exchangeable preferred limited partnership units, number of tranches | tranche						4
RBC | eSpeed
Description Of Business
Shares delivered under common stock transactions (in shares)													2,600,000