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Compensation - Limited Partnership Units with a Post-Termination Payout (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award    
Liability balance acquisition related post-termination units $ 6,800  
BGC Holdings, L.P.    
Share-based Compensation Arrangement by Share-based Payment Award    
Notional value 255,356 $ 257,771
Estimated fair value of the post-termination payout $ 67,839 $ 68,682
Outstanding limited partnership units (in shares) 4,754,973 4,873,040
Outstanding limited partnership units - unvested (in dollars per share) 734,957 873,822
NEWMARK Group Inc Parent    
Share-based Compensation Arrangement by Share-based Payment Award    
Outstanding limited partnership units (in shares) 20,375,058 20,184,716
Outstanding limited partnership units - unvested (in dollars per share) 9,994,672 9,778,078