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Related Party Transactions - Transactions With Executive Officers and Directors (Details) - USD ($)			3 Months Ended			6 Months Ended		12 Months Ended
	Dec. 19, 2019	Oct. 30, 2019	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Related Party Transaction
Fees to related parties			$ 5,205,000		$ 7,222,000	$ 11,017,000	$ 13,947,000
Class A Common Stock
Related Party Transaction
Treasury stock repurchases (in shares)			0	0	1,613,032	0	1,613,032
Shares repurchased price (in dollars per share)			$ 0	$ 0		$ 9.32		$ 9.32
Mr. Lutnick
Related Party Transaction
Number of non-exchangeable holdings approved for conversion (in shares)	552,483
Exchange share price (in dollars per share)	$ 13.61
Value of LPU issued in exchange	$ 7,017,000
Mr. Gosin
Related Party Transaction
Number of non-exchangeable holdings approved for conversion (in shares)	443,872
Exchange share price (in dollars per share)	$ 13.61
Value of LPU issued in exchange	$ 5,637,548
Mr. Merkel \| Class A Common Stock
Related Party Transaction
Treasury stock repurchases (in shares)	132,429	55,193
Shares repurchased price (in dollars per share)	$ 13.61	$ 10.69
Mr. Merkel \| Newmark Holdings PSUs Exchanged
Related Party Transaction
Fees to related parties	$ 3,791,848
Amount withheld for taxes	1,989,483
Stock issued, value	$ 1,802,365
Stock issued (in shares)	132,429
Shares issued (in dollars per share)	$ 13.61
Mr. Rispoli
Related Party Transaction
Number of non-exchangeable holdings approved for conversion (in shares)	5,846
Exchange share price (in dollars per share)	$ 13.61
Value of LPU issued in exchange	$ 74,250
Preferred Units \| Mr. Lutnick
Related Party Transaction
Number of non-exchangeable holdings approved for conversion (in shares)	552,483
Number of share issued for non exchangeable PPSU (in shares)	602,463
Determination price (in dollars per share)	$ 13.25
Payment of withholding tax rate for common stock issue	$ 7,983,000
Preferred Units \| Mr. Gosin
Related Party Transaction
Number of non-exchangeable holdings approved for conversion (in shares)	443,872
Number of share issued for non exchangeable PPSU (in shares)	539,080
Determination price (in dollars per share)	$ 9.95
Payment of withholding tax rate for common stock issue	$ 5,362,452
Preferred Units \| Mr. Merkel
Related Party Transaction
Exchange share price (in dollars per share)	$ 10.61
Preferred Units \| Mr. Merkel \| Newmark Holdings PSUs Exchanged
Related Party Transaction
Number shares exchanged (in shares)	4,222
Preferred Units \| Mr. Merkel \| PSU
Related Party Transaction
Number of non-exchangeable PSU approved cancellation (in shares)	145,464
Preferred Units \| Mr. Merkel \| Non Exchangeable PPSU
Related Party Transaction
Number of non-exchangeable PSU approved cancellation (in shares)	178,179
Preferred Units \| Mr. Rispoli
Related Party Transaction
Number of non-exchangeable holdings approved for conversion (in shares)	5,846
Number of share issued for non exchangeable PPSU (in shares)	4,917
Determination price (in dollars per share)	$ 12.355
Payment of withholding tax rate for common stock issue	$ 60,750