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Long-Term Debt - Additional Information (Detail) - USD ($)										3 Months Ended		9 Months Ended
	Nov. 28, 2018	Nov. 23, 2018	Nov. 06, 2018	Sep. 26, 2018	Sep. 05, 2018	Sep. 04, 2018	Jun. 19, 2018	Dec. 13, 2017	Sep. 08, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term debt										$ 598,565,000		$ 598,565,000		$ 537,926,000
Repayments of credit agreement												7,802,761,000	$ 5,166,612,000
Borrowings from credit agreement												$ 7,511,234,000	5,937,964,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Stated interest rate										4.05%		4.05%
Long-term debt										$ 58,800,000		$ 58,800,000
Debt issuance cost										1,200,000		1,200,000
Amortization of debt issuance costs										100,000		$ 400,000
Maximum revolving credit	$ 250,000,000.0
Credit agreement maturity period	3 years
Line of credit facility, interest rate description												Borrowings under the Credit Facility will bear annual interest equal to, at Newmark’s option, either (a) LIBOR for specified periods, or upon the consent of all Lenders, such other period that is 12 months or less, plus an applicable margin, or (b) a base rate equal to the greatest of (i) the federal funds rate plus 0.5%, (ii) the prime rate as established by the administrative agent, and (iii) one-month LIBOR plus 1.0%.
Revolving Credit Facility [Member] \| Federal Fund Rate [Member]
Debt Instrument [Line Items]
Applicable margin	0.50%
Revolving Credit Facility [Member] \| One Month LIBOR [Member]
Debt Instrument [Line Items]
Applicable margin	1.00%
Revolving Credit Facility [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Applicable margin	2.00%
Revolving Credit Facility [Member] \| LIBOR [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Applicable margin	0.25%
Revolving Credit Facility [Member] \| LIBOR [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Applicable margin	1.25%
Intercompany Credit Agreement [Member]
Debt Instrument [Line Items]
Stated interest rate						6.50%
Borrowings from credit agreement						$ 112,500,000
Interest Expense, Net [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Interest expense										$ 700,000		$ 1,700,000
Term Loan [Member]
Debt Instrument [Line Items]
Interest expense											$ 2,600,000		7,800,000
Maximum revolving credit									$ 575,000,000.0
Credit agreement maturity date												Sep. 08, 2019
Line of credit facility, description												Borrowings under the Term Loan bore interest at either LIBOR or a defined base rate plus an additional margin which ranged from 50 basis points to 325 basis points depending on BGC’s debt rating as determined by S&P and Fitch and whether such loan was a LIBOR loan or a base rate loan.
Repayments of credit agreement								$ 304,300,000				$ 270,700,000
Term Loan [Member] \| LIBOR or Defined Base Rate [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Applicable margin									0.50%
Term Loan [Member] \| LIBOR or Defined Base Rate [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Applicable margin									3.25%
Converted Term Loan [Member]
Debt Instrument [Line Items]
Long-term debt														$ 0
Interest expense											3,100,000		12,100,000
Maximum revolving credit									$ 400,000,000.0
Credit agreement maturity date												Sep. 08, 2019
Line of credit facility, description												Borrowings under the Converted Term Loan bore interest at either LIBOR or a defined base rate plus an additional margin, which ranged from 50 basis points to 325 basis points depending on BGC’s debt rating as determined by S&P and Fitch and whether such loan was a LIBOR loan or a base rate loan, and whether there were amounts outstanding under the Term Loan. The Term Loan was paid in full on March 9, 2018. Since the Term Loan was repaid in full, the pricing of the Converted Term Loan returned to the levels previously described.
Converted Term Loan [Member] \| RBC [Member]
Debt Instrument [Line Items]
Repayments of convertible debt				$ 113,200,000			$ 152,900,000
Converted Term Loan [Member] \| LIBOR or Defined Base Rate [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Applicable margin									0.50%
Converted Term Loan [Member] \| LIBOR or Defined Base Rate [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Applicable margin									3.25%
6.125% Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument face amount			$ 550,000,000.0
Stated interest rate			6.125%							6.125%		6.125%		6.125%
Issued price percentage			98.937%
Yield percentage			6.375%
Debt instrument, payment terms												payable on each May 15 and November 15, beginning on May 15, 2019
Debt instrument, date of first required payment												May 15, 2019
Long-term debt										$ 539,789,000		$ 539,789,000		$ 537,926,000
Maturity date												Nov. 15, 2023
Debt instrument, issuance date												Nov. 01, 2018
Debt issuance cost										5,300,000		$ 5,300,000
Debt discount										4,900,000		4,900,000
Amortization of debt issuance costs										300,000		1,000,000.0
6.125% Senior Notes [Member] \| Interest Expense, Net [Member]
Debt Instrument [Line Items]
Interest expense										8,700,000		26,000,000.0
6.125% Senior Notes [Member] \| Converted Term Loan [Member]
Debt Instrument [Line Items]
Repayments of convertible debt			$ 134,000,000.0
6.125% Senior Notes [Member] \| Straight Line Method [Member]
Debt Instrument [Line Items]
Amortization of debt issuance costs										$ 300,000		$ 800,000
2019 Promissory Note [Member]
Debt Instrument [Line Items]
Stated interest rate								5.375%
Maturity date												Dec. 09, 2019
Interest expense											4,300,000		12,800,000
Promissory Note								$ 300,000,000.0
Repayments of debt		$ 300,000,000.0
Prepayment penalty		$ 7,000,000.0
2042 Promissory Note [Member]
Debt Instrument [Line Items]
Stated interest rate								8.125%
Maturity date												Jun. 26, 2042
Interest expense											$ 1,600,000		$ 6,300,000
Promissory Note								$ 112,500,000
Repayments of debt					$ 112,500,000	$ 112,500,000