Related Party Transactions - Additional Information (Detail)																	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 06, 2020 USD ($) $ / shares shares	Aug. 05, 2020 USD ($) $ / shares shares	Jul. 30, 2020 USD ($) $ / shares shares	Jul. 10, 2020 USD ($)	Mar. 20, 2020 USD ($) $ / shares shares	Mar. 02, 2020 USD ($) shares	Sep. 27, 2019 USD ($)	Mar. 27, 2019 $ / shares shares	Mar. 05, 2019 $ / shares shares	Nov. 30, 2018 shares	Nov. 23, 2018 USD ($) shares	Mar. 09, 2018	Dec. 13, 2017	Oct. 25, 2016 shares	May 27, 2016 USD ($)	Jun. 05, 2015 shares	Jul. 31, 2020 shares	May 31, 2020 USD ($) sqrft	Sep. 30, 2020 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) shares	Sep. 30, 2020 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	Aug. 14, 2020 USD ($)	Jun. 11, 2020 USD ($)	Feb. 05, 2020 USD ($)	Dec. 31, 2019 USD ($) shares	Feb. 27, 2019 USD ($)	Aug. 06, 2018 USD ($)	Mar. 19, 2018 USD ($)	Nov. 04, 2016	Aug. 31, 2013 USD ($)
Related Party Transactions [Abstract]
Repurchase agreements																			$ 2,089,000		$ 2,089,000
Commissions																			352,027,000	$ 409,765,000	1,190,522,000	$ 1,262,921,000
Common stock, issued during the period, additional consideration											$ 0
Interest expense from affiliate																			0	0	400,000	$ 0
Payables to Cantor related to open derivative contracts																			45,750,000		45,750,000						$ 72,497,000
Employee loans																			$ 401,633,000		$ 401,633,000						315,590,000
Securities loaned																											13,902,000
Limited Partnership Units Exchange Ratio																					1
Repurchase of Class A common stock, shares | shares																	2,000		2,259		2,259	233,172
Equity investment																					$ 1,103,000	$ 1,103,000
Principal sum of loan due from related parties																			$ 14,246,000		14,246,000						$ 14,273,000
Aqua [Member]
Related Party Transactions [Abstract]
Increase in authorization amount for loans and investments, other credit support arrangements																										$ 2,000,000.0
Authorization amount for loans and investments, other credit support arrangements																										$ 19,200,000
Equity investment																			500,000	$ 400,000	1,100,000
Principal sum of loan due from related parties																			$ 980,000.0		$ 980,000.0
Maturity date of subordinated loan																					Sep. 01, 2021
Loan Receivable, Description of Variable Rate Basis																					three month LIBOR plus 600 basis points
Loan Receivable, Basis Spread on Variable Rate																			6.00%		6.00%
Stephen M. Merkel [Member]
Related Party Transactions [Abstract]
Number of exchange rights grant | shares						360,065
Number of non-exchangeable LPU | shares						360,065
Shares repurchased at average price of shares | $ / shares					$ 4.0024
Number of shares redeemed, value					$ 741,644
Number of PLPU share issued for non-exchangeable | shares					265,568
Payment of withholding tax rate for common stock issue					$ 1,507,285
Number of non-exchangeable units redeemed | shares			142,989		122,579
Payment related tax for redeemed PLPU			$ 846,182		$ 661,303
Shaun D. Lynn [Member]
Related Party Transactions [Abstract]
Number of exchange rights grant | shares						883,348
Number of non-exchangeable LPU | shares						883,348
Number of PLPU share issued for non-exchangeable | shares						245,140
Payment of withholding tax rate for common stock issue						$ 1,099,599
Number of non-exchangeable units redeemed | shares			221,239
Payment related tax for redeemed PLPU			$ 992,388
Number of remaining exchangeable LPUs | shares			86,126
Number of exchangeable PLPUs | shares			23,901
Payment related to exchangeable LPUs and PLPUs redeemed upon exchange in connection with LLP status			$ 0
Sean A. Windeatt [Member]
Related Party Transactions [Abstract]
Number of exchange rights grant | shares						519,725
Number of non-exchangeable LPU | shares						519,725
Number of shares redeemed | shares		20,849
Number of PLPU share issued for non-exchangeable | shares						97,656
Payment of withholding tax rate for common stock issue						$ 645,779
Number of non-exchangeable units redeemed | shares		96,216
Payment related tax for redeemed PLPU		$ 637,866
Number of exchangeable PLPUs | shares		1,440
Payment related to exchangeable LPUs and PLPUs redeemed upon exchange in connection with LLP status		$ 0
Mr. Windeatt [Member]
Related Party Transactions [Abstract]
Number of exchange rights grant | shares		40,437
Number of non-exchangeable LPU | shares	43,890	40,437
Number of shares redeemed | shares	3,681
Number of PLPU share issued for non-exchangeable | shares	17,068	21,774
Payment related tax for redeemed PLPU	$ 194,086
Number of exchangeable PLPUs | shares	1,431	21,774
Payment related to exchangeable LPUs and PLPUs redeemed upon exchange in connection with LLP status	$ 0	$ 136,305
Transaction charge of grant of exchangeability	$ 381,961
Debt Securities [Member]
Related Party Transactions [Abstract]
Purchased senior notes																			$ 50,000,000.0		$ 50,000,000.0
5.125% Senior Notes Due 2021 [Member]
Related Party Transactions [Abstract]
Principal amount of debt component															$ 300,000,000.0
Stated interest rate															5.125%
3.750% Senior Notes Due October 1, 2024 [Member]
Related Party Transactions [Abstract]
Principal amount of debt component							$ 300,000,000.0
Related party expenses							$ 36,000
Stated interest rate							3.75%
4.375% Senior Notes [Member]
Related Party Transactions [Abstract]
Principal amount of debt component				$ 300,000,000.0
Related party expenses																					$ 36,000
Stated interest rate				4.375%															4.375%		4.375%						4.375%
5.125% Senior Notes [Member]
Related Party Transactions [Abstract]
Principal amount of debt component															$ 300,000,000.0
Stated interest rate																			5.125%		5.125%			5.125%			5.125%
14.66% Senior Notes [Member]
Related Party Transactions [Abstract]
Principal amount of debt component																								$ 44,000,000.0
Stated interest rate																								14.66%
Maximum [Member]
Related Party Transactions [Abstract]
Payment percentage of the gross proceeds from the sale of shares												2.00%
Maximum [Member] | Debt Securities [Member]
Related Party Transactions [Abstract]
Purchased senior notes																									$ 50,000,000.0
BGC Class B Common Stock [Member]
Related Party Transactions [Abstract]
Percentage of Distributions										100.00%
Newmark [Member]
Related Party Transactions [Abstract]
Number of shares, held by parent | shares										15,100,000
Class A Common Stock [Member]
Related Party Transactions [Abstract]
Common stock, shares issued | shares																			365,842,000		365,842,000						358,440,000
Issuance of Class A common stock (net of costs), shares | shares																			91,034	53,810	339,617	162,237
Repurchase of Class A common stock, shares | shares																			2,000	0	2,000	233,000
Average price per share of common stock repurchased | $ / shares																			$ 2.58		$ 2.58	$ 5.30
Class A Common Stock [Member] | Stephen M. Merkel [Member]
Related Party Transactions [Abstract]
Number of exchangeable LPU exchanged for issuance of shares | shares						360,065
Number of shares redeemed | shares			174,765		185,300
Percentage of shares repurchased on exchangeable LPU					1.00%
Exchange share price | $ / shares			$ 2.76
Number of shares to be repurchased | shares								250,000
Repurchase of Class A common stock, shares | shares								233,172
Average price per share of common stock repurchased | $ / shares								$ 5.30
Class A Common Stock [Member] | Shaun D. Lynn [Member]
Related Party Transactions [Abstract]
Number of exchangeable LPU exchanged for issuance of shares | shares						883,348
Number of shares redeemed | shares			797,222
Exchange share price | $ / shares			$ 2.76
Class A Common Stock [Member] | Sean A. Windeatt [Member]
Related Party Transactions [Abstract]
Number of exchangeable LPU exchanged for issuance of shares | shares						519,725
Number of shares redeemed | shares		436,665				436,665
Exchange share price | $ / shares		$ 2.90
Class A Common Stock [Member] | Mr. Windeatt [Member]
Related Party Transactions [Abstract]
Number of exchangeable LPU exchanged for issuance of shares | shares	37,660	40,437
Number of shares redeemed | shares	40,209	40,437
Shares repurchased at average price of shares | $ / shares		$ 2.90
Exchange share price | $ / shares	$ 4.16
Number of exchangeable PLPUs | shares	15,637
Class A Common Stock [Member] | Mr. Lutnick [Member] | Retirement Plan [Member]
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs), shares | shares									8,980
Share Price | $ / shares									$ 6.24
Class A Common Stock [Member] | Maximum [Member] | Mr. Lutnick [Member] | Retirement Plan [Member]
Related Party Transactions [Abstract]
Value of common stock to be issued																												$ 56,038
Class A Common Stock [Member] | Newmark [Member]
Related Party Transactions [Abstract]
Number of shares, right to exchange from Class A to Class B common stock | shares										9,400,000
Class A Common Stock [Member] | Newmark OpCo [Member]
Related Party Transactions [Abstract]
Number of shares, held by parent | shares										7,000,000.0
Number of shares, right to exchange from Class A to Class B common stock | shares										6,900,000
Class B Common Stock [Member]
Related Party Transactions [Abstract]
Common stock, shares issued | shares																			45,884,000		45,884,000						45,884,000
Issuance of Class A common stock (net of costs), shares | shares																			0	0	0	0
Class B Common Stock [Member] | Newmark [Member]
Related Party Transactions [Abstract]
Number of shares, right to exchange from Class A to Class B common stock | shares										5,400,000
Class B Units [Member] | Lucera [Member]
Related Party Transactions [Abstract]
Purchase of units | shares														9,000
Percentage of outstanding common stock owned by parent																															100.00%
Newmark [Member] | LPUs [Member]
Related Party Transactions [Abstract]
Aggregate exchangeable limited partnership units purchased | shares										1,500,000
Newmark [Member] | Class A Common Stock [Member]
Related Party Transactions [Abstract]
Spin-off received on per share										0.463895
Aggregate distribution of shares | shares										131,900,000
Common stock, shares issued | shares										131,886,409
Newmark [Member] | Class B Common Stock [Member]
Related Party Transactions [Abstract]
Spin-off received on per share										0.463895
Aggregate distribution of shares | shares										21,300,000
Percentage of Distributions										100.00%
Common stock, shares issued | shares										21,285,537
Exchange ratio																					0.9373
Tower Bridge International Services LP [Member] | BGC Partners, Inc. [Member]
Related Party Transactions [Abstract]
Company ownership of Tower Bridge																					52.00%
Tower Bridge International Services LP [Member] | Cantor [Member]
Related Party Transactions [Abstract]
Noncontrolling interest																			48.00%		48.00%
Aqua [Member] | BGC Partners, Inc. [Member]
Related Party Transactions [Abstract]
Percentage of ownership interest																			49.00%		49.00%
Equity investment																						$ 1,100,000
Aqua [Member] | Cantor [Member]
Related Party Transactions [Abstract]
Investment ownership percentage																			51.00%		51.00%
Cantor [Member]
Related Party Transactions [Abstract]
Mark-up cost of services																					7.50%
Cantor's share of net profit (loss) in Tower Bridge																			$ 900,000	$ 200,000	$ 1,100,000	500,000
Recognized related party revenues																			8,800,000	8,200,000	20,900,000	21,200,000
Service charges																			17,000,000.0	16,300,000	48,200,000	44,100,000
Compensation to leased employees																			9,400,000	9,300,000	30,000,000.0	27,600,000
Repurchase agreements																			2,100,000		2,100,000						$ 0
Recognition of share of FX gains (losses)																			1,100,000	(500,000)	900,000	400,000
Commissions																			38,000	24,000	122,000	171,000
Investment in asset-backed commercial paper program																			0		0						0
Investment authorization in asset-backed commercial paper program																																$ 350,000,000.0
Reverse purchase agreement outstanding																			$ 0		$ 0						0
Founding partner units remaining | shares																			2,390,731		2,390,731
Cantor [Member] | Lucera [Member]
Related Party Transactions [Abstract]
Recognized related party revenues																			$ 100,000	200,000	$ 400,000	500,000
Cantor [Member] | Revolving Credit Facility [Member]
Related Party Transactions [Abstract]
Principal amount of debt component																													$ 400,000,000.0	$ 250,000,000.0
Credit agreement, description																					The BGC Credit Agreement will mature on the earlier to occur of (a) March 19, 2021, after which the maturity date of the BGC Credit Agreement will continue to be extended for successive one-year periods unless prior written notice of non-extension is given by a lending party to a borrowing party at least six months in advance of such renewal date and (b) the termination of the BGC Credit Agreement by either party pursuant to its terms.
Credit agreement, interest rate description																					The outstanding amounts under the BGC Credit Agreement will bear interest for any rate period at a per annum rate equal to the higher of BGC’s or Cantor’s short-term borrowing rate in effect at such time plus 1.00%
Line of credit facility, outstanding amount																			0		$ 0						0
Interest income from affiliate																				$ 0		$ 0
Cantor [Member] | Newmark [Member] | Class A Common Stock [Member]
Related Party Transactions [Abstract]
Common stock, shares issued | shares										400,000
CF & Co [Member]
Related Party Transactions [Abstract]
Payment percentage of the gross proceeds from the sale of shares													5.50%
Repurchase agreements																			$ 2,100,000		$ 2,100,000						0
Issuance of Class A common stock (net of costs), shares | shares																			0	0	200,000	900,000
Aggregate proceeds from issuance of common stock																					$ 700,000	$ 5,300,000
Weighted-average price per share | $ / shares																					$ 4.04	$ 6.17
Related party expenses																					$ 7,000	$ 100,000
Securities loaned																			$ 0		0						$ 13,900,000
Stated interest rate																											2.45%
Guarantee fees																			31,000	$ 31,000	94,000	94,000
CF & Co [Member] | Underwriting Fees [Member]
Related Party Transactions [Abstract]
Related party expenses																					14,500
CF & Co [Member] | 5.125% Senior Notes Due 2021 [Member] | Underwriting Fees [Member]
Related Party Transactions [Abstract]
Related party expenses															500,000
Purchased senior notes																			$ 15,000,000.0		$ 15,000,000.0
CF & Co [Member] | 3.750% Senior Notes Due October 1, 2024 [Member] | Underwriting Fees [Member]
Related Party Transactions [Abstract]
Related party expenses							$ 200,000
CF & Co [Member] | 4.375% Senior Notes [Member] | Underwriting Fees [Member]
Related Party Transactions [Abstract]
Related party expenses				$ 200,000
CF & Co [Member] | Securities Financing Transaction, Fair Value [Member]
Related Party Transactions [Abstract]
Securities loaned																											$ 13,900,000
CF & Co [Member] | Class A Common Stock [Member]
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs), shares | shares																			0
CF Group Management, Inc. [Member]
Related Party Transactions [Abstract]
Number of shares, right to exchange from Class A to Class B common stock | shares											712,907					34,649,693					23,600,000
Cantor Rights to Purchase Exchangeable Units [Member]
Related Party Transactions [Abstract]
Number of shares, right to exchange from Class A to Class B common stock | shares											10,323,366
Cantor Rights to Purchase Exchangeable Units [Member] | Maximum [Member]
Related Party Transactions [Abstract]
Number of shares, right to exchange from Class A to Class B common stock | shares																34,649,693					23,600,000
Cantor Rights to Purchase Exchangeable Units [Member] | Class B Common Stock [Member] | Maximum [Member]
Related Party Transactions [Abstract]
Number of shares, right to exchange from Class A to Class B common stock | shares																					23,613,420
Cantor and CFGM [Member] | Class B Common Stock [Member]
Related Party Transactions [Abstract]
Number of shares, right to exchange from Class A to Class B common stock | shares																					23,613,420
Cantor and CFGM [Member] | Newmark [Member] | Class A Common Stock [Member]
Related Party Transactions [Abstract]
Common stock, shares issued | shares										449,917
Cantor and Freedom [Member]
Related Party Transactions [Abstract]
Receivables from Freedom																				1,300,000		1,300,000					1,300,000
Receivables from Cantor related to open derivative contracts																				200,000		200,000					1,300,000
Payables to Cantor related to open derivative contracts																				400,000		400,000					2,000,000.0
Payables to Cantor related to pending equity trades																				27,200,000		27,200,000					2,300,000
Employee Loans [Member]
Related Party Transactions [Abstract]
Employee loans																			$ 401,600,000		$ 401,600,000						315,600,000
Compensation expense related to employee loans																			11,100,000	9,300,000	42,700,000	23,300,000
Interest income																			1,700,000	$ 1,300,000	6,100,000	$ 3,500,000
Castle Oak | 5.125% Senior Notes Due 2021 [Member] | Underwriting Fees [Member]
Related Party Transactions [Abstract]
Related party expenses															$ 18,000
Cantor Fitzgerald Relief Fund [Member]
Related Party Transactions [Abstract]
Charitable contributions																			1,100,000		1,100,000
Cantor Fitzgerald Relief Fund [Member] | Accounts Payable and Accrued Liabilities
Related Party Transactions [Abstract]
Charitable contributions																			4,500,000		4,500,000						$ 4,000,000.0
Cantor Fitzgerald Relief Fund [Member] | Other Expenses [Member]
Related Party Transactions [Abstract]
Charitable contributions																							$ 40,000,000.0
RKF Retail Holdings LLC [Member]
Related Party Transactions [Abstract]
Lessee operating lease sublease term																		1 year
Area of sublease | sqrft																		21,000
Lessee operating sublease description																		In May 2020, BGC U.S. OpCo entered into an arrangement to sublease excess space from RKF Retail Holdings LLC, a subsidiary of Newmark, which sublease was approved by the Audit Committee. The deal is a one-year sublease of approximately 21,000 rentable square feet in New York City.
BGC U.S OpCo [Member]
Related Party Transactions [Abstract]
Rent payment under lease term																		$ 1,100,000	$ 400,000		$ 500,000