Related Party Transactions - Additional Information (Detail)										1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 30, 2019 $ / shares shares	Jul. 22, 2019 USD ($)	Feb. 06, 2019 USD ($) $ / shares shares	Nov. 30, 2018 USD ($) shares	May 01, 2018 USD ($) Employee	Mar. 19, 2018	Mar. 07, 2018 USD ($) $ / shares shares	Dec. 13, 2017	Sep. 08, 2017 USD ($)	Feb. 28, 2019 USD ($)	Oct. 31, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) shares	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) $ / shares shares	Aug. 16, 2019	Feb. 22, 2019 shares	Nov. 06, 2018 USD ($)	Dec. 31, 2017 USD ($)
Related Party Transaction [Line Items]
Fees to related parties												$ 7,088,000	$ 6,644,000	$ 21,035,000	$ 19,839,000
Loans, forgivable loans and other receivables from employees and partners												356,590,000		356,590,000		$ 285,532,000
Compensation and employee benefits												397,683,000	332,158,000	1,030,997,000	944,668,000
Limited partnership units issued															265,569,000
Stockholders equity												610,375,000		610,375,000		567,569,000
Redeemable partnership interests												28,895,000		28,895,000		26,170,000				$ 21,096,000
Mortgage servicing right recognized												34,908,000	23,288,000	70,112,000	58,286,000
Management services, servicing fees and other												156,394,000	147,497,000	457,692,000	423,217,000
Determination price | $ / shares			$ 4.2625
Non-exchangeable units redemption, value																$ 11,000,000
Exchange share price | $ / shares			$ 6.20													$ 5.17
Value of LPU issued in exchange			$ 9,870,501
Payment of withholding tax rate for common stock issue			$ 1,129,499
Payment related to withholding tax rate for common shares issued																$ 64,471
Number od shares removed from sale restriction | shares																		4,229
Equity method investments												$ 99,966,000		$ 99,966,000		101,275,000
Exchange Ratio Number of Common Shares Received Under Spin Off Transaction												93.62%		93.62%
Purchase of units | shares							16,600,000
Purchase value of units							$ 242,000,000.0
Related party receivables												$ 0.0		$ 0.0		20,500,000
Current portion of payables to related parties												$ 19,771,000		$ 19,771,000		$ 13,507,000
6.125% Senior Notes [Member]
Related Party Transaction [Line Items]
Debt instrument face amount																			$ 550,000,000.0
Stated interest rate												6.125%		6.125%		6.125%			6.125%
CF Real Estate Finance Holdings, L.P. [Member]
Related Party Transaction [Line Items]
Equity method investments									$ 100,000,000.0
Class A Common Stock [Member]
Related Party Transaction [Line Items]
Aggregate distribution of shares in spin-off | shares				131,900,000
Limited partnership units exchange ratio														0.9362
Class B Common Stock [Member]
Related Party Transaction [Line Items]
Aggregate distribution of shares in spin-off | shares				21,300,000
Class B Common Stock [Member] | Cantor And CFGM [Member]
Related Party Transaction [Line Items]
Percentage of Distributions of shares in spin-off				100.00%
Preferred Units [Member]
Related Party Transaction [Line Items]
Number of non-exchangeable units redeemed | shares			1,592,016													592,721
Determination price | $ / shares																$ 13.715
Non-exchangeable units redemption, value																$ 15,000,000
CCRE Lending [Member]
Related Party Transaction [Line Items]
Percentage of third party bank purchased loan value																	80.00%
Percentage of third party bank retaining loan value																	20.00%
Loan maturity date		Aug. 06, 2029
Loans receivable, fixed interest rate		4.38%
Loans receivable, brokerage service amount		$ 700,000
CCRE Lending [Member] | Commercial Real Estate Loan [Member]
Related Party Transaction [Line Items]
Loans receivable, commercial real estate loan amount		$ 146,600,000
BGC Holdings, L.P. [Member]
Related Party Transaction [Line Items]
Number of non-exchangeable units redeemed | shares			1,131,774													264,985
Number of non-exchangeable psu approved redemption | shares			1,131,774
Exchange share price | $ / shares			$ 6.20
Value of LPU issued in exchange			$ 7,017,000
Number of share issued for non exchangeable PPSU | shares			1,018,390													134,535
Number of net share issued for non exchangeable units redemption | shares																13,552
Exchange Ratio Number of Common Shares Received Under Spin Off Transaction				46.3895%
Non-Exchangeable PPSU [Member]
Related Party Transaction [Line Items]
Determination price | $ / shares			$ 7.8388
Payment of withholding tax rate for common stock issue			$ 7,983,000
BGC Partners Inc [Member] | Class A or Class B Common Stock [Member]
Related Party Transaction [Line Items]
Exchange Ratio Number of Common Shares Received Under Spin Off Transaction				46.3895%
BGC Partners Inc [Member] | Class B Common Stock [Member] | Cantor And CFGM [Member]
Related Party Transaction [Line Items]
Percentage of common shares held by limited partners				100.00%
Chief Executive Officer [Member] | CCRE Lending [Member]
Related Party Transaction [Line Items]
Percentage of ownership interest of loan receivable		19.00%
Mr. Lutnick [Member] | BGC Holdings, L.P. [Member] | Preferred Units [Member]
Related Party Transaction [Line Items]
Number of non-exchangeable units redeemed | shares																898,080,000
Determination price | $ / shares																$ 7.65
Mr. Gosin [Member]
Related Party Transaction [Line Items]
Determination price | $ / shares																$ 4.2625
Mr. Gosin [Member] | BGC Holdings, L.P. [Member]
Related Party Transaction [Line Items]
Number of share issued for non exchangeable PPSU | shares			264,985
Mr. Gosin [Member] | BGC Holdings, L.P. [Member] | Preferred Units [Member]
Related Party Transaction [Line Items]
Number of non-exchangeable units redeemed | shares																1,909,188
Mr. Rispoli [Member]
Related Party Transaction [Line Items]
Determination price | $ / shares																$ 5.814
Number of non-exchangeable PSU approved cancellation | shares																13,552,000
Number od shares removed from sale restriction | shares																		1,961
Mr. Rispoli [Member] | BGC Holdings, L.P. [Member]
Related Party Transaction [Line Items]
Number of non-exchangeable PSU approved cancellation | shares																11,089,000
Mr. Merkel [Member] | Class A Common Stock [Member] | Subsequent Event [Member]
Related Party Transaction [Line Items]
Stock repurchase during period, shares | shares	55,193
Stock price per share | $ / shares	$ 10.69
BPF [Member]
Related Party Transaction [Line Items]
Total consideration transferred									875,000,000.0
Business acquisition price paid in units									$ 3,200,000
Business acquisition date									Sep. 08, 2017
Business acquisition, transaction agreement date									Jul. 17, 2017
Intercompany Credit Agreement [Member]
Related Party Transaction [Line Items]
Interest rate						1.00%
Interest rate terms														The interest rate on the Intercompany Credit Agreement was 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.
Intercompany Credit Agreement [Member] | Interest Expense, Net [Member]
Related Party Transaction [Line Items]
Interest expense													4,200,000		7,800,000
Cantor and BGC [Member]
Related Party Transaction [Line Items]
Fees to related parties												$ 7,100,000	6,600,000	$ 21,000,000.0	19,800,000
Recognized related party revenues												100,000	300,000	800,000	300,000
Employee Loans [Member]
Related Party Transaction [Line Items]
Loans, forgivable loans and other receivables from employees and partners												356,600,000		356,600,000		$ 285,500,000
Compensation and employee benefits												13,200,000	9,500,000	31,700,000	20,700,000
CCRE [Member]
Related Party Transaction [Line Items]
Fees to related parties												0	0	0	0
Number of employees transferred | Employee					5
Proceeds from limited partnership units											$ 6,900,000
Issuance of additional limited partnership units with capital account														2,200,000
Issuance of additional limited partners units in exchange for cash payment														$ 500,000
Issuance of additional limited partners units in exchange | shares														2,200,000
Maximum revenue share to related party in first two years					$ 3,300,000
Stockholders equity												6,900,000		$ 6,900,000
Redeemable partnership interests												2,200,000		2,200,000
CCRE [Member] | BPF [Member]
Related Party Transaction [Line Items]
Transaction agreement, distribution									$ 89,100,000
Net assets exceeded									$ 508,600,000
CCRE [Member] | Loan Referral Agreement [Member]
Related Party Transaction [Line Items]
Revenue from loan referral agreement												700,000	1,500,000	1,600,000	2,200,000
Broker fees and commissions												200,000	33,000	400,000	800,000
CCRE [Member] | Primary Servicing Rights [Member]
Related Party Transaction [Line Items]
Servicing rights of loans originated												584,000,000.0	905,000,000	584,000,000.0	905,000,000
Purchase of loans originated														800,000	1,500,000
Mortgage servicing right recognized														0
Management services, servicing fees and other												$ 900,000	$ 900,000	2,900,000	$ 2,800,000
CCRE [Member] | Newmark Holdings, L.P. [Member]
Related Party Transaction [Line Items]
Limited partnership units issued											6,700,000
Limited partnership unit cash distributions											$ 200,000
Cantor and its Affiliates (Other Than BGC) [Member] | GSE Loans [Member]
Related Party Transaction [Line Items]
Related party transaction maximum amount per loan										$ 100,000,000.0
Related party transaction limit on loan not yet acquired or sold										250,000,000.0
Cantor and its Affiliates (Other Than BGC) [Member] | GSE Loans [Member] | Fannie Mae [Member]
Related Party Transaction [Line Items]
Related party transaction limit on loan outstanding										$ 250,000,000.0
CF & Co [Member]
Related Party Transaction [Line Items]
Percentage of gross proceeds paid to underwriter								5.50%
Related party transaction, service fees paid														$ 4,000,000.0
Stated interest rate												2.33%		2.33%
CF & Co [Member] | Cantor Credit Agreement [Member]
Related Party Transaction [Line Items]
Maximum revolving credit				$ 250,000,000
Line of credit facility, interest rate description				an interest rate which is the higher of Cantor’s or Newmark’s short-term borrowing rate then in effect, plus 1%.
CF & Co [Member] | 6.125% Senior Notes [Member]
Related Party Transaction [Line Items]
Debt instrument face amount																			$ 550,000,000.0
Stated interest rate																			6.125%
Debt instrument, underwriting fees paid to CF&Co																			$ 800,000
BGC Partners, L.P. and its Operating Subsidiaries [Member] | Newmark Units [Member]
Related Party Transaction [Line Items]
Purchase of units | shares							16,600,000
Purchase value of units							$ 242,000,000.0
Closing price per share | $ / shares							$ 14.57
Limited partners capital account units held by BGC | shares							14,800,000
Number of limited partnership units exchangeable | shares				1,500,000
Limited partnership units exchange ratio							0.9793							0.9362
BGC Partners, L.P. and its Operating Subsidiaries [Member] | Class A Common Stock [Member] | Newmark Units [Member]
Related Party Transaction [Line Items]
Number of shares, right to exchange from Class A to Class A or Class B common stock | shares							9,400,000
BGC Partners, L.P. and its Operating Subsidiaries [Member] | Class B Common Stock [Member] | Newmark Units [Member]
Related Party Transaction [Line Items]
Number of shares, right to exchange from Class A to Class A or Class B common stock | shares							5,400,000
BGC Partners, L.P. and its Operating Subsidiaries [Member] | Cantor [Member] | Newmark Units [Member]
Related Party Transaction [Line Items]
Aggregate distribution of shares in spin-off | shares							400,000
BGC Partners, L.P. and its Operating Subsidiaries [Member] | Newmark OpCo [Member] | Newmark Units [Member]
Related Party Transaction [Line Items]
Limited partnership interests units | shares							7,000,000.0
Limited partners capital account units held by BGC | shares							7,000,000.0
BGC Partners, L.P. and its Operating Subsidiaries [Member] | Newmark OpCo [Member] | Class A Common Stock [Member] | Newmark Units [Member]
Related Party Transaction [Line Items]
Number of shares, right to exchange from Class A to Class A or Class B common stock | shares							6,900,000