Related Party Transactions - Transactions With Executive Officers and Directors (Details) - USD ($)				3 Months Ended				12 Months Ended
	Dec. 19, 2019	Oct. 30, 2019	Feb. 06, 2019	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Feb. 22, 2019
Related Party Transaction [Line Items]
Fees to related parties								$ 25,025,000	$ 26,162,000	$ 20,771,000
Stock issued, value										$ 295,419,000
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		$ 6.20
Value of LPU issued in exchange	$ 7,017,000		$ 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
Number of non-exchangeable units redeemed (in shares)			1,592,016
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
Number of shares removed from sale restriction (in shares)											1,961
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								$ 13.715
Non-exchangeable units redemption, value									$ 15,000,000
Payment of withholding tax rate for common stock issue	$ 7,983,000
Number of non-exchangeable units redeemed (in shares)									592,721
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		$ 4.2625
Non-exchangeable units redemption, value									$ 11,000,000
Payment of withholding tax rate for common stock issue	$ 5,362,452		$ 1,129,499
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
BGC Holdings, L.P. | Mr. Lutnick
Related Party Transaction [Line Items]
Number of share issued for non exchangeable PPSU (in shares)			1,018,390
Determination price (in dollars per share)			$ 7.8388
Payment of withholding tax rate for common stock issue			$ 7,983,000
Number of non-exchangeable PSUs approved redemption (in shares)			1,131,774
BGC Holdings, L.P. | Mr. Gosin
Related Party Transaction [Line Items]
Exchange share price (in dollars per share)			$ 6.20
Value of LPU issued in exchange			$ 9,870,501
BGC Holdings, L.P. | Mr. Rispoli
Related Party Transaction [Line Items]
Exchange share price (in dollars per share)									$ 5.17
Number of share issued for non exchangeable PPSU (in shares)									134,535
Number of net share issued for non exchangeable units redemption (in shares)									13,552
Payment related to withholding tax rate for common shares issued									$ 64,471
Number of shares removed from sale restriction (in shares)											4,229
BGC Holdings, L.P. | Preferred units | Mr. Lutnick
Related Party Transaction [Line Items]
Determination price (in dollars per share)									$ 7.65
Number of non-exchangeable units redeemed (in shares)			1,131,774						898,100,000
BGC Holdings, L.P. | Preferred units | Mr. Gosin
Related Party Transaction [Line Items]
Number of share issued for non exchangeable PPSU (in shares)			264,985
Determination price (in dollars per share)									$ 4.2625
Number of non-exchangeable units redeemed (in shares)									1,909,188
Number of non-exchangeable PSU approved cancellation (in shares)									264,985
BGC Holdings, L.P. | Preferred units | Mr. Rispoli
Related Party Transaction [Line Items]
Determination price (in dollars per share)									$ 5.814
BGC Holdings, L.P. | Preferred units | Mr. Rispoli | Non Exchangeable PPSU
Related Party Transaction [Line Items]
Number of non-exchangeable PSU approved cancellation (in shares)									11,089
BGC Holdings, L.P. | Preferred units | Mr. Rispoli | PSU
Related Party Transaction [Line Items]
Number of non-exchangeable PSU approved cancellation (in shares)									13,552
Class A Common Stock
Related Party Transaction [Line Items]
Stock issued (in shares)										23,000,000
Treasury stock repurchases (in shares)				625,597	2,279,373	1,613,032	0	4,518,002	50,000
Shares repurchased price (in dollars per share)				$ 12.97	$ 8.81	$ 8.61	$ 0	$ 9.32	$ 9.73
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