Related Party Transactions - Additional Information (Detail)															1 Months Ended	3 Months Ended		9 Months Ended	12 Months Ended
	Mar. 08, 2021 USD ($)	Aug. 14, 2020 USD ($)	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 ($) shares	Sep. 27, 2019 USD ($)	Nov. 23, 2018 USD ($) shares	Jul. 24, 2018 USD ($)	Oct. 25, 2016 shares	May 27, 2016 USD ($)	Jun. 05, 2015 shares	May 31, 2020 USD ($) sqrft	Mar. 31, 2021 USD ($) shares	Mar. 31, 2020 USD ($) $ / shares shares	Sep. 30, 2020 USD ($)	Dec. 31, 2015 USD ($)	Feb. 25, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 11, 2020 USD ($)	Feb. 05, 2020 USD ($)	Aug. 06, 2018 USD ($)	Mar. 19, 2018 USD ($)	Nov. 04, 2016	Aug. 31, 2013 USD ($)
Related Party Transactions [Abstract]
Repurchase agreements																$ 94,000
Commissions																435,220,000	$ 455,855,000
Common stock, issued during the period, additional consideration										$ 0
Interest expense from affiliate																0	0
Payables to Cantor related to open derivative contracts																47,501,000					$ 36,252,000
Employee loans																$ 419,638,000					408,142,000
Limited Partnership Units Exchange Ratio																1
Equity investment																$ 355,000	245,000
Receivables from related parties																6,919,000					$ 11,953,000
Aqua [Member]
Related Party Transactions [Abstract]
Increase in authorization amount for loans and investments, other credit support arrangements																				$ 1,000,000.0			$ 2,000,000.0
Authorization amount for loans and investments, other credit support arrangements																				$ 20,200,000			$ 20,200,000
Equity investment																400,000	$ 200,000
Receivables from related parties																$ 980,000.0
Maturity date of subordinated loan																Sep. 01, 2022
Loan Receivable, Description of Variable Rate Basis																three month LIBOR plus 600 basis points
Loan Receivable, Basis Spread on Variable Rate																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
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%
5.375% Senior Notes Due 2023 [Member]
Related Party Transactions [Abstract]
Principal amount of debt component											$ 450,000,000.0
Stated interest rate											5.375%
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%
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%
14.66% Senior Notes [Member]
Related Party Transactions [Abstract]
Principal amount of debt component		$ 44,000,000.0
Stated interest rate		14.66%
Class B Common Stock [Member]
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs), shares | shares																0	0
Class A Common Stock [Member]
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs), shares | shares																262,373	71,663
Aggregate proceeds from issuance of common stock	$ 300,000,000.0
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
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 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%
Maximum [Member] | Debt Securities [Member]
Related Party Transactions [Abstract]
Purchased senior notes																						$ 50,000,000.0
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%
Aqua [Member] | BGC Partners, Inc. [Member]
Related Party Transactions [Abstract]
Percentage of ownership interest																49.00%
Aqua [Member] | Cantor [Member]
Related Party Transactions [Abstract]
Investment ownership percentage																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																$ 40,000	$ (134,000)
Recognized related party revenues																3,800,000	5,500,000
Service charges																21,100,000	15,900,000
Compensation to leased employees																15,800,000	10,400,000
Repurchase agreements																100,000					$ 0
Recognition of share of FX gains																100,000	1,700,000
Commissions																25,000	75,000
Investment in asset-backed commercial paper program																0	0
Investment authorization in asset-backed commercial paper program																											$ 350,000,000.0
Reverse purchase agreement outstanding																$ 0					0
Founding partner units remaining | shares																500,000
Cantor [Member] | Lucera [Member]
Related Party Transactions [Abstract]
Recognized related party revenues																$ 100,000	100,000
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, 2022, 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
CF Group Management, Inc. [Member]
Related Party Transactions [Abstract]
Number of shares, right to exchange from Class A to Class B common stock | shares										700,000				34,600,000		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										10,300,000
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														34,600,000		23,600,000
Cantor Rights to Purchase Exchangeable Units [Member] | Maximum [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																23,600,000
Cantor Rights to Purchase Exchangeable Units [Member] | BGC Holdings LPUs [Member]
Related Party Transactions [Abstract]
Aggregate cantor units as result of redemption | shares																1,149,684
Aggregate consideration of cantor units as result of redemption																$ 2,104,433,000
Number of founding partner interests as result of redemption | shares																1,149,684
Aggregate cantor units | shares																1,618,376
Aggregate consideration of cantor units																$ 3,040,411,000
Number of founding partner interests exchanged | shares																1,618,376
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																23,600,000
Cantor and Freedom [Member]
Related Party Transactions [Abstract]
Receivables from Freedom																$ 2,200,000					1,400,000
Receivables from Cantor related to open derivative contracts																2,800,000					600,000
Payables to Cantor related to open derivative contracts																1,500,000					100,000
Payables to Cantor related to pending equity trades																11,000,000.0					26,000,000.0
Employee Loans [Member]
Related Party Transactions [Abstract]
Employee loans																419,600,000					408,100,000
Compensation expense related to employee loans																15,600,000	14,600,000
Interest income																2,200,000	$ 2,700,000
CF & Co [Member]
Related Party Transactions [Abstract]
Repurchase agreements																$ 100,000					0
Issuance of Class A common stock (net of costs), shares | shares																0	200,000
Aggregate proceeds from issuance of common stock																	$ 700,000
Weighted-average price per share | $ / shares																	$ 4.04
Related party expenses																	$ 7,000
Securities loaned																$ 0					0
Guarantee fees																31,000	$ 31,000
CF & Co [Member] | Underwriting Fees [Member]
Related Party Transactions [Abstract]
Related party expenses		$ 14,000
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
CF & Co [Member] | 5.375% Senior Notes Due 2023 [Member]
Related Party Transactions [Abstract]
Advisory fees											$ 200,000
CF & Co [Member] | 5.375% Senior Notes Due 2023 [Member] | Underwriting Fees [Member]
Related Party Transactions [Abstract]
Related party expenses											300,000
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
Purchased senior notes																$ 14,500,000
CF & Co [Member] | Class A Common Stock [Member]
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs), shares | shares																0
Castle Oak | 5.125% Senior Notes Due 2021 [Member] | Underwriting Fees [Member]
Related Party Transactions [Abstract]
Related party expenses													$ 18,000
Castle Oak | 5.375% Senior Notes Due 2023 [Member] | Underwriting Fees [Member]
Related Party Transactions [Abstract]
Related party expenses											$ 41,000
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																1,600,000					$ 1,600,000
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	$ 300,000