Organization and Basis of Presentation (Details) $ in Thousands					12 Months Ended
	Nov. 30, 2020 USD ($) shares	Dec. 02, 2019 USD ($) shares	Sep. 26, 2018 USD ($) tranche shares	Jun. 28, 2013 USD ($) shares	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)	Nov. 30, 2019 USD ($)	Jan. 01, 2019 USD ($)	Jun. 18, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 13, 2017
Description Of Business
Shares owned (in shares) | shares	250,742	93,562
Proceeds from issuance of EPUs					$ 0	$ 0	$ 262,169
Investment owned shares settled fair value	$ 93,500	$ 93,500
Shares settled (in shares) | shares	741,505	898,685
Marketable securities					33,283	36,795
Assets					3,982,450	3,201,599
Total liabilities					3,041,258	2,239,457
Retained earnings					342,764	313,112
Noncontrolling interests					266,098	340,961
Right-of-use assets					190,469	201,661			$ 178,800
Operating lease liability					$ 248,097	255,126			$ 226,700
Cumulative Effect, Period of Adoption, Adjustment
Description Of Business
Assets						(8,000)					$ 64,400
Total liabilities						17,900					45,600
Retained earnings						$ (25,900)					16,500
Noncontrolling interests											$ 2,300
Nasdaq Omx
Description Of Business
Purchase consideration paid in cash				$ 750,000
Period for expected payment under Common stock transaction (years)				15 years
Nasdaq Omx | Maximum
Description Of Business
Shares received from transaction (in shares) | shares			992,247
Common Class A
Description Of Business
Limited partnership units exchange ratio (in percentage)												1
Common stock, shares issued (in shares) | shares					167,604,348	160,833,463
Common Class B
Description Of Business
Common stock, shares issued (in shares) | shares					21,285,533	21,285,533
BGC Partners Inc | Nasdaq Omx | Maximum
Description Of Business
Expected payment of shares under common stock transaction (in shares) | shares				14,883,705
BGC Partners Inc | Nasdaq Omx | Minimum
Description Of Business
Gross revenue on expected payment per year under common stock transaction				$ 25,000
Newmark OpCo | RBC
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
Common Stock
Description Of Business
Fair value of equity security					$ 33,300	$ 36,800
Common Stock | Nasdaq Earn-Out
Description Of Business
Fair value of equity security	$ 121,900
Marketable securities								$ 98,600