Organization and Basis of Presentation (Details) $ in Thousands					1 Months Ended	12 Months Ended
	Dec. 02, 2019 USD ($) shares	Nov. 30, 2018 shares	Mar. 07, 2018 USD ($) shares	Jun. 28, 2013 USD ($) shares	Nov. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)	Mar. 31, 2020 USD ($) tranche shares	Mar. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)	Sep. 26, 2018 USD ($) tranche contract	Jun. 18, 2018 USD ($)	Dec. 13, 2017
Description Of Business [Line Items]
Marketable securities					$ 98,600	$ 36,795		$ 0
Exchangeable Preferred Partnership Units reclassified					$ 93,500
Shares settled (in shares) | shares	898,685
Shares settled fair value	$ 93,500
Shares owned (in shares) | shares	93,562
Number of units converted to common stock (in shares) | shares		14,800,000
Ownership interest percentage		94.00%
Economic interest ownership percentage		87.00%
Common stock units issued (in shares) | shares		1,458,931
Right-of-use assets						201,661		195,510
Operating lease liability						255,126		249,520
Assets						(3,201,599)		(3,886,237)
Liabilities						2,239,457		2,943,925
Stockholders equity						(940,625)	$ (1,056,799)	(920,933)	$ (1,020,798)
Retained earnings						$ 313,112		$ 282,053
Class A Common Stock
Description Of Business [Line Items]
Limited partnership interest spinoff ratio		46.3895%
Number of units converted to common stock (in shares) | shares		9,400,000
Common stock, shares issued (in shares) | shares		131,886,409				160,833,463		161,269,748
Common stock units issued (in shares) | shares		449,917
Limited partnership units exchange ratio								0.9461					1
Class B Common Stock
Description Of Business [Line Items]
Limited partnership interest spinoff ratio		46.3895%
Number of units converted to common stock (in shares) | shares		7,000,000.0
Common stock, shares issued (in shares) | shares		21,285,537				21,285,533		21,285,533
BGC Partners LP and its Operating Subsidiaries | Newmark Units
Description Of Business [Line Items]
Purchase of units | shares			16,600,000
Purchase value of units			$ 242,000
Limited partnership units exchange ratio								0.9461
Accounting Standards Update 2016-02
Description Of Business [Line Items]
Right-of-use assets										$ 178,800
Operating lease liability										$ 226,700
Nasdaq Omx
Description Of Business [Line Items]
Purchase consideration paid in cash				$ 750,000
Period for expected payment under Common stock transaction				15 years
Maximum | Nasdaq Omx
Description Of Business [Line Items]
Shares received from transaction (in shares) | shares						992,247
Maximum | Nasdaq Omx | BGC Partners Inc
Description Of Business [Line Items]
Expected payment of shares under common stock transaction | shares				14,883,705
Minimum | Nasdaq Omx | BGC Partners Inc
Description Of Business [Line Items]
Gross revenue on expected payment per year under common stock transaction.				$ 25,000
RBC
Description Of Business [Line Items]
Proceeds from issuance of EPUs							266,100
Newmark OpCo
Description Of Business [Line Items]
Number of units converted to common stock (in shares) | shares		7,000,000
Newmark OpCo | Class A Common Stock
Description Of Business [Line Items]
Number of units converted to common stock (in shares) | shares		6,900,000
Newmark OpCo | Class B Common Stock
Description Of Business [Line Items]
Number of units converted to common stock (in shares) | shares		5,400,000
Newmark OpCo | RBC
Description Of Business [Line Items]
Exchangeable preferred limited partnership units issued (in shares)											$ 150,000	$ 175,000
Exchangeable preferred limited partnership units, number of tranches | tranche								4			4
Retained Earnings
Description Of Business [Line Items]
Stockholders equity						$ (313,112)	$ (277,952)	$ (282,053)	$ (275,589)
Nasdaq Forwards
Description Of Business [Line Items]
Number of derivative contracts | contract											4
Cumulative Effect, Period of Adoption, Adjustment
Description Of Business [Line Items]
Assets						8,000
Liabilities						17,900
Stockholders equity						19,023
Retained earnings						(25,900)
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
Description Of Business [Line Items]
Stockholders equity						$ 19,023