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Related Party Transactions - Transactions With Executive Officers and Directors (Details) - USD ($)			3 Months Ended		12 Months Ended	20 Months Ended
	Dec. 19, 2019	Oct. 30, 2019	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Mar. 31, 2020
Related Party Transaction [Line Items]
Fees to related parties			$ 5,812,000	$ 6,725,000
Mr. Lutnick
Related Party Transaction [Line Items]
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 [Line Items]
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 \| Newmark Holdings PSUs Exchanged
Related Party Transaction [Line Items]
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 [Line Items]
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 [Line Items]
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 [Line Items]
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 [Line Items]
Exchange share price (in dollars per share)	$ 10.61
Preferred Units \| Mr. Merkel \| Newmark Holdings PSUs Exchanged
Related Party Transaction [Line Items]
Number shares exchanged (in shares)	4,222
Preferred Units \| Mr. Merkel \| Non Exchangeable PPSU
Related Party Transaction [Line Items]
Number of non-exchangeable PSU approved cancellation (in shares)	178,179
Preferred Units \| Mr. Merkel \| PSU
Related Party Transaction [Line Items]
Number of non-exchangeable PSU approved cancellation (in shares)	145,464
Preferred Units \| Mr. Rispoli
Related Party Transaction [Line Items]
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
Class A Common Stock
Related Party Transaction [Line Items]
Treasury stock repurchases (in shares)			0
Shares repurchased price (in dollars per share)			$ 0		$ 9.32	$ 9.32
Class A Common Stock \| Mr. Merkel
Related Party Transaction [Line Items]
Treasury stock repurchases (in shares)	132,429	55,193
Shares repurchased price (in dollars per share)	$ 13.61	$ 10.69