Related Party Transactions - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions						3 Months Ended		12 Months Ended
	Mar. 19, 2018	Mar. 07, 2018	Dec. 13, 2017	Sep. 08, 2017	Mar. 11, 2015	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Fees to related parties						$ 6,894,000	$ 4,718,000
Employee loans						226,744,000		$ 209,549,000
Total consideration transferred								55,600,000
Mortgage servicing right recognized						17,817,000	15,952,000
Equity method investments						104,718,000		101,562,000
Receivables from related parties								0
Current portion of payables to related parties						$ 197,199,000		34,169,000
Intercompany Credit Agreement [Member]
Related Party Transaction [Line Items]
Interest rate	1.00%
Interest rate terms						The interest rate on the Intercompany Credit Agreement can be the higher of BGC’s or Newmark’s short term borrowings rate in effect at such time plus 100 basis points, or such other interest rate as may be mutually agreed between BGC and Newmark.
Stated interest rate						4.99%
Line of credit facility, outstanding amount						$ 202,000,000
CF Real Estate Finance Holdings, L.P. [Member]
Related Party Transaction [Line Items]
Equity method investments				$ 100,000,000
Loan to related parties						339,200,000
Note Receivable/Payable [Member]
Related Party Transaction [Line Items]
Interest expense						2,100,000
One Month LIBOR [Member] | Note Receivable/Payable [Member]
Related Party Transaction [Line Items]
Interest rate					1.00%
BPF [Member]
Related Party Transaction [Line Items]
Total consideration transferred				875,000,000
Business acquisition price paid in units				$ 3,200,000
Business acquisition date				Sep. 08, 2017
Business acquisition, transaction agreement date				Jul. 17, 2017
Net assets exceeded				$ 508,600,000
Gains from Mortgage Banking Activities, Net [Member]
Related Party Transaction [Line Items]
Broker fees and commissions						700,000	200,000
Interest Income, Net [Member] | Intercompany Credit Agreement [Member]
Related Party Transaction [Line Items]
Interest expense						1,000,000
Loan Referral Agreement [Member] | Gains from Mortgage Banking Activities, Net [Member]
Related Party Transaction [Line Items]
Revenue from loan referral agreement						3,700,000	300,000
Cantor and BGC [Member]
Related Party Transaction [Line Items]
Fees to related parties						6,900,000		4,700,000
Employee Loans [Member]
Related Party Transaction [Line Items]
Employee loans						226,700,000		$ 209,600,000
Compensation expense related to employee loans						6,000,000	2,000,000
CCRE [Member]
Related Party Transaction [Line Items]
Fees to related parties						0	0
Recognized related party revenues						0	60,900,000
CCRE [Member] | Primary Servicing Rights [Member]
Related Party Transaction [Line Items]
Servicing rights of loans originated						300,000,000	0
Purchase of loans originated						500,000
Mortgage servicing right recognized						0
Servicing fees						$ 900,000	$ 1,000,000
Cantor and CCRE and Cantor Commercial Real Estate Sponsor, L.P. [Member] | BPF [Member]
Related Party Transaction [Line Items]
Transaction agreement, distribution				89,100,000
Net assets exceeded				$ 508,600,000
CF & Co [Member]
Related Party Transaction [Line Items]
Percentage of gross proceeds paid to underwriter			5.50%
Stated interest rate						2.18%
BGC Partners LP and its Operating Subsidiaries [Member] | Newmark Units [Member]
Related Party Transaction [Line Items]
Purchase of units		16.6
Purchase value of units		$ 242,000,000
Closing price per share		$ 14.57