Related Party Transactions - Additional Information (Detail)																							1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 30, 2021 USD ($)	Jun. 28, 2021 USD ($) $ / shares shares	Jun. 24, 2021	Apr. 29, 2021 USD ($) shares	Apr. 23, 2021 USD ($) $ / shares shares	Mar. 31, 2021 USD ($) shares	Mar. 08, 2021 USD ($)	Feb. 22, 2021 USD ($) shares	Aug. 14, 2020 USD ($)	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 shares	Sep. 27, 2019 USD ($)	Nov. 23, 2018 USD ($) shares	Aug. 06, 2018 USD ($)	Jul. 24, 2018 USD ($)	Oct. 25, 2016 shares	May 27, 2016 USD ($)	Jun. 05, 2015 shares	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($)	May 31, 2020 USD ($) ft²	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	May 31, 2021 USD ($)	Apr. 08, 2021 shares	Feb. 25, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 11, 2020 USD ($)	Feb. 05, 2020 USD ($)	Nov. 28, 2018	Mar. 19, 2018 USD ($)	Nov. 04, 2016	Aug. 31, 2013 USD ($)
Related Party Transactions [Abstract]
Repurchase agreements																							$ 2,997,000			$ 2,997,000		$ 2,997,000						$ 0
Commissions																										367,016,000	$ 352,027,000	1,192,004,000	$ 1,190,522,000
Payables to Cantor related to open derivative contracts																							93,560,000			93,560,000		93,560,000						36,921,000
Employee loans																							375,982,000			375,982,000		375,982,000						408,142,000
Number of H units redeemed (in shares) | shares		1,131,774
Payment related to H units redeemed		$ 7,017,000
Number of preferred H units redeemed (in shares) | shares		1,018,390
Payment related tax for redeemed preferred H units		$ 7,983,000
Equity investment																												870,000	1,103,000
Receivables from related parties																							6,124,000			6,124,000		6,124,000						11,915,000
Futures Exchange Group
Related Party Transactions [Abstract]
Payments to acquire businesses	$ 4,900,000
Aqua
Related Party Transactions [Abstract]
Increase in authorization amount for loans and investments, other credit support arrangements																																	$ 1,000,000			$ 2,000,000
Authorization amount for loans and investments, other credit support arrangements																																	$ 20,200,000
Equity investment																										200,000	$ 500,000	900,000	$ 1,100,000
Receivables from related parties																							$ 980,000			$ 980,000		$ 980,000
Loan receivable, basis spread																							6.00%			6.00%		6.00%
Stephen M. Merkel
Related Party Transactions [Abstract]
Number of PLPU share issued for non-exchangeable (in shares) | shares														265,568
Payment of withholding tax rate for common stock issue														$ 1,507,285
Number of shares redeemed, value														$ 741,644
Number of non-exchangeable units redeemed (in shares) | shares				108,350								142,989		122,579
Number of non-exchangeable PSUs (in shares) | shares				273,612
Payment related to non-exchangeable PSUs redeemed upon exchange				$ 0
Number of non-exchangeable PPSUs redeemed | shares				101,358
Number of non-exchangeable PPSUs (in shares) | shares				250,659
Payments related to non-exchangeable PPSUs redeemed upon exchange				$ 575,687
Payment related tax for redeemed PLPU												$ 846,182		$ 661,303
Shares repurchased at average price of shares (in dollars per share) | $ / shares														$ 4.0024
Mr. Lutnick | Exchange LPUs For Common Stock
Related Party Transactions [Abstract]
Number of non-exchangeable LPU (in shares) | shares		88,636
Number of exchangeable LPU exchanged for issuance of shares (in shares) | shares		520,380
Exchange share price (in dollars per share) | $ / shares		$ 5.86
Number of shares redeemed (in shares) | shares		0
Mr. Lutnick | Exchange PLPUs for Common Stock
Related Party Transactions [Abstract]
Number of exchangeable PLPUs (in shares) | shares		425,765
Payment related tax for redeemed PLPU		$ 1,525,705
Shaun D. Lynn
Related Party Transactions [Abstract]
Number of exchange rights grant (in shares) | shares															883,348
Number of PLPU share issued for non-exchangeable (in shares) | shares															245,140
Payment of withholding tax rate for common stock issue															$ 1,099,599
Number of non-exchangeable units redeemed (in shares) | shares												221,239
Number of exchangeable PLPUs (in shares) | shares												23,901
Payment related tax for redeemed PLPU												$ 992,388
Number of remaining exchangeable LPUs (in shares) | shares												86,126
Payment related to exchangeable LPUs and PLPUs redeemed upon exchange in connection with LLP status															$ 0
Sean A. Windeatt
Related Party Transactions [Abstract]
Number of exchange rights grant (in shares) | shares								123,713							519,725
Number of non-exchangeable LPU (in shares) | shares								123,713							519,725
Number of PLPU share issued for non-exchangeable (in shares) | shares								28,477							97,656
Payment of withholding tax rate for common stock issue								$ 178,266							$ 645,779
Shares authorized to be repurchased (in shares) | shares					123,713
Exchange share price (in dollars per share) | $ / shares					$ 5.65										$ 5.38
Number of shares redeemed (in shares) | shares					28,477						20,849
Number of shares redeemed, value					$ 178,266
Remaining number of shares authorized to be repurchased (in shares) | shares																																62,211
Number of non-exchangeable units redeemed (in shares) | shares											96,216
Number of exchangeable PLPUs (in shares) | shares											1,440
Payment related tax for redeemed PLPU											$ 637,866
Payment related to exchangeable LPUs and PLPUs redeemed upon exchange in connection with LLP status															$ 0
Mr. Windeatt
Related Party Transactions [Abstract]
Number of exchange rights grant (in shares) | shares											40,437
Number of non-exchangeable LPU (in shares) | shares										43,890	40,437
Number of PLPU share issued for non-exchangeable (in shares) | shares										17,068	21,774
Number of shares redeemed (in shares) | shares										3,681
Number of exchangeable PLPUs (in shares) | shares										1,431	21,774
Payment related tax for redeemed PLPU										$ 194,086
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
Related Party Transactions [Abstract]
Debt instrument, authorized repurchase amount																																			$ 50,000,000
Debt instrument, remaining authorized repurchase amount																							$ 50,000,000			$ 50,000,000		$ 50,000,000
5.125% Senior Notes Due 2021
Related Party Transactions [Abstract]
Principal amount of debt component																					$ 300,000,000
Stated interest rate																					5.125%
5.375% Senior Notes Due 2023
Related Party Transactions [Abstract]
Principal amount of debt component																			$ 450,000,000
Stated interest rate																			5.375%
3.750% Senior Notes Due October 1, 2024
Related Party Transactions [Abstract]
Principal amount of debt component																$ 300,000,000
Stated interest rate																3.75%
4.375% Senior Notes
Related Party Transactions [Abstract]
Principal amount of debt component													$ 300,000,000
Stated interest rate													4.375%										4.375%			4.375%		4.375%
5.125% Senior Notes due May 27, 2021
Related Party Transactions [Abstract]
Principal amount of debt component											$ 300,000,000										$ 300,000,000
Stated interest rate																					5.125%		5.125%			5.125%		5.125%
14.66% Senior Notes
Related Party Transactions [Abstract]
Principal amount of debt component									$ 44,000,000
Stated interest rate									14.66%
Class B Common Stock
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs) (in shares) | shares																										0	0	0	0
Class A Common Stock
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs) (in shares) | shares																										100,128	91,034	366,043	339,617
Aggregate proceeds from issuance of common stock							$ 300,000,000
Stock repurchased during period (in shares) | shares																										24,400,000	2,000	41,900,000	2,000
Class A Common Stock | Stephen M. Merkel
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs) (in shares) | shares				108,350
Number of exchange rights grant (in shares) | shares															360,065
Number of non-exchangeable LPU (in shares) | shares															360,065
Number of exchangeable LPU exchanged for issuance of shares (in shares) | shares															360,065
Exchange share price (in dollars per share) | $ / shares												$ 2.76
Number of shares redeemed (in shares) | shares												174,765		185,300
Stock repurchased during period (in shares) | shares				108,350
Percentage of shares repurchased on exchangeable LPU														1.00%
Class A Common Stock | Mr. Lutnick | Exchange LPUs For Common Stock
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs) (in shares) | shares		520,380
Common stock, shares issued net of shares for tax withholdings (in shares) | shares		365,229
Class A Common Stock | Mr. Lutnick | Exchange PLPUs for Common Stock
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs) (in shares) | shares		88,636
Common stock, shares issued net of shares for tax withholdings (in shares) | shares		41,464
Class A Common Stock | Shaun D. Lynn
Related Party Transactions [Abstract]
Number of non-exchangeable LPU (in shares) | shares															883,348
Number of exchangeable LPU exchanged for issuance of shares (in shares) | shares															883,348
Exchange share price (in dollars per share) | $ / shares												$ 2.76
Number of shares redeemed (in shares) | shares												797,222
Class A Common Stock | Sean A. Windeatt
Related Party Transactions [Abstract]
Number of exchangeable LPU exchanged for issuance of shares (in shares) | shares								123,713							519,725
Exchange share price (in dollars per share) | $ / shares											$ 2.90
Number of shares redeemed (in shares) | shares											436,665
Class A Common Stock | Mr. Windeatt
Related Party Transactions [Abstract]
Number of exchangeable LPU exchanged for issuance of shares (in shares) | shares										37,660	40,437
Exchange share price (in dollars per share) | $ / shares										$ 4.16
Number of shares redeemed (in shares) | shares										40,209	40,437
Number of exchangeable PLPUs (in shares) | shares										15,637
Shares repurchased at average price of shares (in dollars per share) | $ / shares											$ 2.90
Class B Units | Lucera
Related Party Transactions [Abstract]
Purchase of units (in shares) | shares																				9,000
Percentage of outstanding common stock owned by parent																																							100.00%
Cantor
Related Party Transactions [Abstract]
Repurchase agreements																							$ 3,000,000			$ 3,000,000		$ 3,000,000						0
Futures Exchange Group | Cantor
Related Party Transactions [Abstract]
Related party transaction, contributed capital																										$ 0	$ 600,000	$ 5,300,000	$ 4,600,000
Tower Bridge International Services LP | BGC Partners, Inc.
Related Party Transactions [Abstract]
Company ownership of Tower Bridge																												52.00%
Tower Bridge International Services LP | Cantor
Related Party Transactions [Abstract]
Ownership percentage by noncontrolling owners																							48.00%			48.00%		48.00%
Aqua | BGC Partners, Inc.
Related Party Transactions [Abstract]
Percentage of ownership interest																							49.00%			49.00%		49.00%
Aqua | Cantor
Related Party Transactions [Abstract]
Investment ownership percentage																							51.00%			51.00%		51.00%
Cantor
Related Party Transactions [Abstract]
Mark-up cost of services																												7.50%
Cantor's share of net profit (loss) in Tower Bridge																										$ 700,000	900,000	$ 1,300,000	1,100,000
Recognized related party revenues																										3,500,000	8,800,000	11,500,000	20,900,000
Service charges																										18,700,000	17,000,000	60,000,000	48,200,000
Compensation to leased employees																										13,000,000	9,400,000	44,400,000	30,000,000
Recognition of share of FX gains																										200,000	1,100,000	200,000	900,000
Commissions																										24,000	38,000	86,000	122,000
Investment authorization in asset-backed commercial paper program																																								$ 350,000,000
Investment in asset-backed commercial paper program																							$ 0			0		0						0
Common stock, issued during the period, additional consideration																	$ 0
Reverse purchase agreement outstanding																							0			0		0						0
Cantor | Futures Exchange Group
Related Party Transactions [Abstract]
Business combination, indemnified expenses (up to)	1,000,000
Indemnification assets	$ 400,000
Cantor | Lucera
Related Party Transactions [Abstract]
Recognized related party revenues																										100,000	100,000	300,000	400,000
Cantor | Revolving Credit Facility
Related Party Transactions [Abstract]
Principal amount of debt component																		$ 400,000,000																				$ 250,000,000
Debt instrument, extended term																		1 year
Debt instrument, non-extension notice term																		6 months
Debt instrument, basis spread on variable rate														1.00%
Line of credit facility, outstanding amount																							$ 0			0		0						0
Interest expense from affiliate																										$ 0	0	$ 0	400,000
CF Group Management, Inc.
Related Party Transactions [Abstract]
Right to exchange from Class A to Class B common stock (up to) (in shares) | shares																	700,000					34,600,000
Common stock, issued during the period, additional consideration																	$ 0
Cantor Rights to Purchase Exchangeable Units
Related Party Transactions [Abstract]
Right to exchange from Class A to Class B common stock (up to) (in shares) | shares																	10,300,000											23,600,000
Limited partnership units, exchange ratio																												1
Cantor Rights to Purchase Exchangeable Units | Maximum
Related Party Transactions [Abstract]
Right to exchange from Class A to Class B common stock (up to) (in shares) | shares																						34,600,000						23,600,000
Cantor Rights to Purchase Exchangeable Units | BGC Holdings LPUs
Related Party Transactions [Abstract]
Limited partnership units, exchange ratio						1
Aggregate cantor units as result of redemption | shares						1,149,684
Aggregate consideration of cantor units as result of redemption						$ 2,104,433
Number of founding partner interests as result of redemption (in shares) | shares						1,149,684
Aggregate cantor units (in shares) | shares						1,618,376
Aggregate consideration of cantor units						$ 3,040,411
Number of founding partner interests exchanged (in shares) | shares						1,618,376
Founding partner units remaining (in shares) | shares																							1,600,000			1,600,000		1,600,000
Cantor and CFGM | Conversion of Class B Common Stock to Class A Common Stock
Related Party Transactions [Abstract]
Common stock, conversion ratio																						1															1
Cantor and CFGM | Class B Common Stock
Related Party Transactions [Abstract]
Right to exchange from Class A to Class B common stock (up to) (in shares) | shares																												23,600,000
Cantor and Freedom
Related Party Transactions [Abstract]
Receivables from Freedom																							$ 1,400,000			$ 1,400,000		$ 1,400,000						1,400,000
Receivables from Cantor related to open derivative contracts																							3,900,000			3,900,000		3,900,000						600,000
Payables to Cantor related to open derivative contracts																							5,000,000			5,000,000		5,000,000						100,000
Payables to Cantor related to pending equity trades																							18,100,000			18,100,000		18,100,000						26,000,000
Employee Loans
Related Party Transactions [Abstract]
Employee loans																							376,000,000			376,000,000		376,000,000						408,100,000
Compensation expense related to employee loans																										11,100,000	11,100,000	45,700,000	42,700,000
Interest income																										2,000,000	$ 1,700,000	7,500,000	$ 6,100,000
Employee Loans | Disposal Group, Held-for-sale, Not Discontinued Operations
Related Party Transactions [Abstract]
Employee loans																							13,900,000			13,900,000		13,900,000
CF & Co
Related Party Transactions [Abstract]
Repurchase agreements																							3,000,000			$ 3,000,000		3,000,000						0
Issuance of Class A common stock (net of costs) (in shares) | shares																										0	0		200,000
Aggregate proceeds from issuance of common stock																													$ 700,000
Weighted-average price (in dollars per share) | $ / shares																													$ 4.04
Related party expenses																										$ 0		0	$ 7,000
Securities loaned																							0			0		0						0
Guarantee fees																										31,000	$ 31,000	94,000	94,000
CF & Co | Underwriting Fees
Related Party Transactions [Abstract]
Related party expenses									$ 14,000
CF & Co | 5.125% Senior Notes Due 2021 | Underwriting Fees
Related Party Transactions [Abstract]
Related party expenses																					$ 500,000
Purchased senior notes									$ 15,000,000
CF & Co | 5.375% Senior Notes Due 2023
Related Party Transactions [Abstract]
Advisory fees																			$ 200,000
CF & Co | 5.375% Senior Notes Due 2023 | Underwriting Fees
Related Party Transactions [Abstract]
Related party expenses																			$ 300,000
CF & Co | 4.375% Senior Notes
Related Party Transactions [Abstract]
Purchased senior notes																							14,500,000			14,500,000		14,500,000
CF & Co | 4.375% Senior Notes | Underwriting Fees
Related Party Transactions [Abstract]
Related party expenses													$ 200,000
Castle Oak | 3.750% Senior Notes Due October 1, 2024 | Underwriting Fees
Related Party Transactions [Abstract]
Related party expenses																$ 200,000
Cantor Fitzgerald Relief Fund
Related Party Transactions [Abstract]
Additional expense associated with charitable contributions																							8,300,000			8,300,000		8,300,000						1,100,000
Charitable contributions, additional expense recognized																							7,200,000	$ 1,100,000
Cantor Fitzgerald Relief Fund | Accounts Payable and Accrued Liabilities
Related Party Transactions [Abstract]
Remaining liability associated with charitable contributions																							$ 1,700,000			1,700,000		1,700,000						$ 1,600,000
Cantor Fitzgerald Relief Fund | Other Expenses
Related Party Transactions [Abstract]
Charitable contributions																														$ 40,000,000
RKF Retail Holdings LLC
Related Party Transactions [Abstract]
Lessee operating lease sublease term																									1 year
Area of sublease | ft²																									21,000
BGC U.S OpCo
Related Party Transactions [Abstract]
Operating lease cost																									$ 1,100,000
Rent payment under lease term																										$ 100,000	$ 400,000	$ 500,000	$ 500,000
Aurel BGC SAS | Cantor
Related Party Transactions [Abstract]
Percentage of revenue shared between related parties			80.00%
Revenue sharing agreement, term			12 months
Poten & Partners
Related Party Transactions [Abstract]
Monthly rent expense																															$ 15,000