Organization and Basis of Presentation (Details) $ in Thousands						1 Months Ended	12 Months Ended
	Dec. 02, 2019 USD ($) shares	Nov. 30, 2018 shares	Sep. 26, 2018 USD ($) tranche contract shares	Mar. 07, 2018 USD ($) shares	Jun. 28, 2013 USD ($) shares	Nov. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Jan. 01, 2019 USD ($)	Nov. 29, 2018 shares	Jun. 18, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 13, 2017
Description Of Business
Common stock units issued (in shares) | shares		1,458,931
Marketable securities						$ 98,600		$ 0	$ 36,795
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
Assets								(4,132,238)	(3,201,599)
Liabilities								3,226,183	2,239,457
Retained earnings								(278,130)	(313,112)
Noncontrolling interests								312,099	340,961
Right-of-use assets								191,177	201,661
Operating lease liability								$ 244,905	$ 255,126
Class A Common Stock
Description Of Business
Limited partnership units exchange ratio (in percentage)								0.9366						1
Common stock, shares issued (in shares) | shares								162,379,438	160,833,463
Common stock units issued (in shares) | shares		449,917
Class B Common Stock
Description Of Business
Common stock, shares issued (in shares) | shares								21,285,533	21,285,533
Newmark Units | BGC Partners LP and its Operating Subsidiaries
Description Of Business
Purchase of units (units) | shares				16,600,000
Purchase value of units				$ 242,000
Accounting Standards Update 2016-02
Description Of Business
Right-of-use assets										$ 178,800
Operating lease liability										$ 226,700
Nasdaq Omx
Description Of Business
Purchase consideration paid in cash					$ 750,000
Period for expected payment under Common stock transaction (years)					15 years
Maximum | Nasdaq Omx
Description Of Business
Shares received from transaction (in shares) | shares			992,247
Maximum | Nasdaq Omx | BGC Partners Inc
Description Of Business
Expected payment of shares under common stock transaction (in shares) | shares					14,883,705
Minimum | Nasdaq Omx | BGC Partners Inc
Description Of Business
Gross revenue on expected payment per year under common stock transaction					$ 25,000
RBC | Newmark OpCo
Description Of Business
Exchangeable preferred limited partnership units issued (in shares)			$ 150,000									$ 175,000
Proceeds from issuance of EPUs							$ 266,100
Exchangeable preferred limited partnership units, number of tranches | tranche			4
Nasdaq Forwards
Description Of Business
Number of derivative contracts (contracts) | contract			4
Cumulative effect of the credit loss standard adoption, net of tax
Description Of Business
Assets									$ 8,000				$ (64,400)
Liabilities									17,900				45,600
Retained earnings									$ 25,900				(16,500)
Noncontrolling interests													$ 2,300
Spinoff | Class A Common Stock
Description Of Business
Limited partnership units exchange ratio (in percentage)		0.463895
Number of units converted to common stock (in shares) | shares		9,400,000
Spinoff | Class B Common Stock
Description Of Business
Limited partnership units exchange ratio (in percentage)		0.463895
Number of units converted to common stock (in shares) | shares		5,400,000
Spinoff | Newmark Holdings | BGC Partners Inc
Description Of Business
Shares owned (in shares) | shares											14,800,000
Spinoff | Newmark OpCo | Class A Common Stock
Description Of Business
Number of units converted to common stock (in shares) | shares		6,900,000
Spinoff | Newmark OpCo | Common Stock
Description Of Business
Number of units converted to common stock (in shares) | shares		7,000,000.0
Spinoff | Newmark Group | BGC Partners Inc | Class A Common Stock
Description Of Business
Common stock, shares issued (in shares) | shares		131,886,409
Ownership interest (percentage)		94.00%
Economic interest ownership percentage (in percentage)		87.00%
Spinoff | Newmark Group | BGC Partners Inc | Class B Common Stock
Description Of Business
Common stock, shares issued (in shares) | shares		21,285,537