Related Party Transactions - Additional Information (Detail)																1 Months Ended	3 Months Ended		12 Months Ended
	Mar. 27, 2019 $ / shares shares	Mar. 05, 2019 $ / shares shares	Feb. 01, 2019 USD ($) $ / shares shares	Nov. 30, 2018 shares	Nov. 23, 2018 USD ($) shares	Jul. 24, 2018 USD ($)	Mar. 07, 2018 USD ($) $ / shares shares	Feb. 26, 2018 $ / shares shares	Dec. 13, 2017	May 27, 2017	Apr. 12, 2017	Nov. 04, 2016 USD ($)	Oct. 25, 2016 shares	May 27, 2016 USD ($)	Jun. 05, 2015 shares	Mar. 31, 2019 USD ($) $ / shares shares	Mar. 31, 2019 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	Mar. 26, 2019 shares	Feb. 27, 2019 USD ($)	Aug. 06, 2018 USD ($)	Mar. 19, 2018 USD ($)	Feb. 16, 2018 USD ($)	Sep. 08, 2017 USD ($)	Apr. 21, 2017 USD ($)	Oct. 27, 2015 USD ($)	Aug. 31, 2013 USD ($)
Related Party Transactions [Abstract]
Cantor's share of net profit (loss) in Tower Bridge																	$ 25,306,000	$ 28,674,000
Payment percentage of the gross proceeds from the sale of shares											2.00%
Equity investment																	368,000	123,000
Repurchase agreements																$ 4,327,000	4,327,000		$ 986,000
Commissions																	429,520,000	407,857,000
Common stock, issued during the period, additional consideration					$ 0
Principal sum of loan due from related parties																11,229,000	11,229,000		7,748,000
Interest expense																	13,198,000	9,368,000
Interest expense from affiliate																	0
Payables to Cantor related to open derivative contracts																38,132,000	38,132,000		40,155,000
Employee loans																240,283,000	240,283,000		216,868,000
Interest income																	3,665,000	2,675,000
Aggregate proceeds from issuance of common stock																		246,704,000
Securities loaned																$ 25,068,000	$ 25,068,000		$ 15,140,000
Limited Partnership Units Exchange Ratio																	1
Repurchase of Class A common stock, shares | shares																233,172	233,172
Average price per share of common stock repurchased | $ / shares																$ 5.30	$ 5.30
Mr. Lutnick [Member] | Preferred Units [Member]
Related Party Transactions [Abstract]
Determination price | $ / shares																			$ 6.57
Number of non-exchangeable units redeemed | shares			376,651																760,797
Non-exchangeable units redemption, value																			$ 5,000,000
Number of non-exchangeable PSU approved redemption | shares			376,651
Exchange share price | $ / shares			$ 6.21
Number of LPU issued on exchangeable			$ 2,339,000,000
Number of PLPU share issued for non exchangeable | shares			463,969
Payment of witholding tax rate for common stock issue			$ 2,661,000
Mr. Merkel [Member]
Related Party Transactions [Abstract]
Share Price | $ / shares																			$ 5.17
Number of non-exchangeable PSU approved cancellation | shares																			113,032
Number of non-exchangeable PPSU approved cancellation | shares																			89,225
Determination price | $ / shares																			$ 5.36
Number of shares issued for non-exchangeable units																			$ 1,062,500,000
Number of shares issued for non-exchangeable units, net | shares																			113,032
Percentage of withholding tax rate for common shares issued																			45.00%
Payment related to withholding tax rate for common shares issued																			$ 478,123
Mr. Windeatt [Member]
Related Party Transactions [Abstract]
Determination price | $ / shares																			$ 4.388
Exchange share price | $ / shares																			$ 5.17
Number of LPU issued on exchangeable																			$ 720,000
Number of PLPU share issued for non exchangeable | shares																			63,814
Payment of witholding tax rate for common stock issue																			$ 280,002
Number of non-exchangeable LPU redemption | shares																			139,265
Mr. Lynn [Member]
Related Party Transactions [Abstract]
Determination price | $ / shares																			$ 3.894
Number of non-exchangeable PSU approved redemption | shares																			750,308
Exchange share price | $ / shares																			$ 5.17
Number of LPU issued on exchangeable																			$ 3,879,092
Number of PLPU share issued for non exchangeable | shares																			287,888
Payment of witholding tax rate for common stock issue																			$ 1,120,909
5.125% Senior Notes Due 2021 [Member]
Related Party Transactions [Abstract]
Principal amount of debt component														$ 300,000,000
Stated interest rate														5.125%		5.125%	5.125%		5.125%
Debt maturity year										2021							2021
5.375% Senior Notes Due 2023 [Member]
Related Party Transactions [Abstract]
Principal amount of debt component						$ 450,000,000
Stated interest rate						5.375%										5.375%	5.375%		5.375%
Debt maturity year						2023
Aqua [Member]
Related Party Transactions [Abstract]
Equity investment																	$ 400,000	$ 100,000
Principal sum of loan due from related parties																$ 980,000	$ 980,000
Increase in authorization amount for loans and investments, other credit support arrangements																												$ 4,000,000
Authorization amount for loans and investments, other credit support arrangements																												$ 16,200,000
Maturity date of subordinated loan																	Sep. 01, 2020
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%
BGC Class B Common Stock [Member]
Related Party Transactions [Abstract]
Percentage of Distributions				1.00
Newmark [Member]
Related Party Transactions [Abstract]
Number of shares, held by parent | shares				15,100,000
Newmark [Member] | Limited Partnership Units [Member]
Related Party Transactions [Abstract]
Aggregate exchangeable limited partnership units purchased | shares							16,600,000
Aggregate consideration amount							$ 242,000,000
Number of interest units owned in limited partnership | shares																7,000,000	7,000,000
Class A Common Stock [Member]
Related Party Transactions [Abstract]
Common stock, shares issued | shares																343,885,000	343,885,000		341,745,000
Issuance of Class A common stock (net of costs), shares | shares																	61,642	18,925,702
Repurchase of Class A common stock, shares | shares																	233,172	0
Average price per share of common stock repurchased | $ / shares																	$ 5.30
Class A Common Stock [Member] | Stephen M. Merkel [Member]
Related Party Transactions [Abstract]
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] | 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 [Member] | Limited Partnership Units [Member]
Related Party Transactions [Abstract]
Share Price | $ / shares							$ 14.57
Class A Common Stock [Member] | Newmark OpCo [Member]
Related Party Transactions [Abstract]
Number of shares, held by parent | shares				7,000,000
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
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%
Cantor and ELX [Member]
Related Party Transactions [Abstract]
Cantor's share of net profit (loss) in Tower Bridge																	$ (300,000)	$ 300,000
Retirement Plan [Member] | Class A Common Stock [Member] | Mr. Lutnick [Member]
Related Party Transactions [Abstract]
Share Price | $ / shares		$ 6.24						$ 13.17
Issuance of Class A common stock (net of costs), shares | shares		8,980						7,883
Retirement Plan [Member] | Class A Common Stock [Member] | Maximum [Member] | Mr. Lutnick [Member]
Related Party Transactions [Abstract]
Value of common stock to be issued																						$ 56,038			$ 105,000
Newmark [Member] | Limited Partnership Units [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	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	0.463895
Aggregate distribution of shares | shares				21,300,000
Percentage of Distributions				1.00
Common stock, shares issued | shares				21,285,537
Exchange ratio				0.9558
Tower Bridge [Member] | BGC Partners, Inc. [Member]
Related Party Transactions [Abstract]
Company ownership of Tower Bridge																	52.00%
Tower Bridge [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%
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%
Repurchase agreements																$ 4,300,000	$ 4,300,000		$ 1,000,000
Recognition of share of foreign exchange gains (losses)																	200,000	(1,000,000)
Commissions																	78,000	58,000
Investment in asset-backed commercial paper program																0	0		0
Investment authorization in asset-backed commercial paper program																													$ 350,000,000
Maximum revolving credit																											$ 150,000,000
Principal sum of loan due from related parties																											$ 150,000,000
Line of credit facility, outstanding amount																0	0		0							$ 150,000,000
Interest income from affiliate																	0	0
Reverse purchase agreement outstanding																$ 0	$ 0		0
Securities loaned																			15,100,000
Founding partner units remaining | shares																1,472,398	1,472,398
Cantor [Member] | Revolving Credit Facility [Member]
Related Party Transactions [Abstract]
Interest income from affiliate																	$ 0
Principal amount of debt component																							$ 400,000,000	$ 250,000,000
Credit agreement, description																	The BGC Credit Agreement will mature on the earlier to occur of (a) March 19, 2020, 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							$ 150,000,000
Interest expense																		400,000
Interest expense related discontinued operation																		400,000
Cantor [Member] | Lucera [Member]
Related Party Transactions [Abstract]
Recognized related party revenues																	100,000	200,000
Cantor [Member] | Class B Units [Member] | Lucera [Member]
Related Party Transactions [Abstract]
Equity investment												$ 24,200,000
Post-closing adjustment												$ 4,800,000
Percentage of ownership interest												20.00%
Cantor [Member] | Newmark [Member] | Class A Common Stock [Member]
Related Party Transactions [Abstract]
Common stock, shares issued | shares				449,917
Cantor and ELX [Member]
Related Party Transactions [Abstract]
Recognized related party revenues																	5,800,000	6,300,000
Service charges																	12,800,000	12,100,000
Compensation to leased employees																	9,900,000	8,100,000
CF & Co [Member]
Related Party Transactions [Abstract]
Payment percentage of the gross proceeds from the sale of shares									5.50%
Repurchase agreements																4,300,000	4,300,000		1,000,000
Securities loaned																25,100,000	25,100,000		$ 15,100,000
Stated interest rate																			2.90%
Guarantee fees																	31,300	$ 31,300
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	15,000,000
CF & Co [Member] | 5.375% Senior Notes Due 2023 [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																$ 25,300,000	$ 25,300,000		$ 15,300,000
CF & Co [Member] | Maximum [Member]
Related Party Transactions [Abstract]
Stated interest rate																3.25%	3.25%
CF & Co [Member] | Minimum [Member]
Related Party Transactions [Abstract]
Stated interest rate																2.95%	2.95%
CF & Co [Member] | March 2018 Sales Agreement [Member]
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs), shares | shares																	900,000
Aggregate proceeds from issuance of common stock																	$ 5,300,000
Weighted-average price per share | $ / shares																	$ 6.17
Related party expenses																	$ 100,000
CF & Co [Member] | April 2017 Sales Agreement [Member]
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs), shares | shares																		19,900,000
Aggregate proceeds from issuance of common stock																		$ 280,400,000
Weighted-average price per share | $ / shares																		$ 14.11
Related party expenses																		$ 5,600,000
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												23,613,420
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 Freedom International Brokerage [Member]
Related Party Transactions [Abstract]
Receivables from Freedom International Brokerage																$ 1,600,000	$ 1,600,000		1,400,000
Receivables from Cantor related to open derivative contracts																2,600,000	2,600,000		1,600,000
Payables to Cantor related to open derivative contracts																2,000,000	2,000,000		2,100,000
Payables to Cantor related to pending equity trades																0	0		0
Employee Loans [Member]
Related Party Transactions [Abstract]
Employee loans																240,300,000	240,300,000		216,900,000
Compensation expense related to employee loans																	6,600,000	1,600,000
Interest income																	1,000,000	$ 700,000
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] | Accounts Payable, Accrued and Other Liabilities [Member]
Related Party Transactions [Abstract]
Charitable contributions																$ 14,800,000	$ 14,800,000		$ 20,000,000
Cantor Fitzgerald Relief Fund [Member] | Other Expenses [Member]
Related Party Transactions [Abstract]
Charitable contributions																				$ 40,000,000