Related Party Transactions - Additional Information (Detail)																			3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 27, 2019 USD ($)	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	Oct. 03, 2018 $ / shares shares	Jul. 24, 2018 USD ($)	Mar. 19, 2018 USD ($)	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	Sep. 30, 2019 USD ($) shares	Mar. 31, 2019 $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	Aug. 08, 2019 USD ($)	Feb. 27, 2019 USD ($)	Aug. 06, 2018 USD ($)	Feb. 16, 2018 USD ($)	Aug. 31, 2013 USD ($)
Related Party Transactions [Abstract]
Cantor's share of net profit (loss) in Tower Bridge																			$ 6,089,000		$ 7,956,000	$ 39,549,000	$ 48,988,000
Payment percentage of the gross proceeds from the sale of shares														2.00%
Equity investment																						1,103,000	599,000
Repurchase agreements																			1,895,000			1,895,000		$ 986,000
Commissions																			409,082,000		351,969,000	1,259,571,000	1,138,313,000
Common stock, issued during the period, additional consideration						$ 0
Interest expense																			15,258,000		10,722,000	43,441,000	30,118,000
Interest income from affiliate																						0
Interest expense from affiliate																			0			0
Payables to Cantor related to open derivative contracts																			97,836,000			97,836,000		40,155,000
Employee loans																			266,920,000			266,920,000		216,868,000
Interest income																			3,976,000		$ 2,870,000	15,454,000	10,485,000
Aggregate proceeds from issuance of common stock																							$ 327,624,000
Securities loaned																			13,000,000			$ 13,000,000		15,140,000
Limited Partnership Units Exchange Ratio																						1
Repurchase of Class A common stock, shares | shares																				233,172	8,263	233,172	734,802
Average price per share of common stock repurchased | $ / shares																				$ 5.30		$ 5.30
Principal sum of loan due from related parties																			$ 9,094,000			$ 9,094,000		$ 7,748,000
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
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 | shares																								1,062,500
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																	$ 300,000,000.0
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								$ 450,000,000.0
Stated interest rate								5.375%											5.375%			5.375%		5.375%
Debt maturity year								2023
3.750% Senior Notes Due October 1, 2024 [Member]
Related Party Transactions [Abstract]
Principal amount of debt component	$ 300,000,000.0
Stated interest rate	3.75%																		3.75%			3.75%		3.75%
Debt maturity year	2024
Aqua [Member]
Related Party Transactions [Abstract]
Equity investment																			$ 400,000		$ 200,000	$ 1,100,000	$ 600,000
Increase in authorization amount for loans and investments, other credit support arrangements																										$ 1,000,000.0
Authorization amount for loans and investments, other credit support arrangements																										$ 17,200,000
Principal sum of loan due from related parties																			$ 980,000			$ 980,000
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					100.00%
Newmark [Member]
Related Party Transactions [Abstract]
Number of shares, held by parent | shares					15,100,000
Newmark [Member] | LPUs [Member]
Related Party Transactions [Abstract]
Aggregate exchangeable limited partnership units purchased | shares										16,600,000
Aggregate consideration amount										$ 242,000,000.0
Number of interest units owned in limited partnership | shares																			7,000,000.0			7,000,000.0
Class A Common Stock [Member]
Related Party Transactions [Abstract]
Common stock, shares issued | shares																			352,840,000			352,840,000		341,745,000
Issuance of Class A common stock (net of costs), shares | shares																			53,810		23,767	162,237	25,975,240
Repurchase of Class A common stock, shares | shares																			0		8,263	233,172	734,802
Average price per share of common stock repurchased | $ / shares																					$ 11.48	$ 5.30	$ 13.35
Class A Common Stock [Member] | Charitable Foundation [Member]
Related Party Transactions [Abstract]
Share Price | $ / shares							$ 11.73
Number of shares repurchased from charitable foundation | shares							53,368
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] | Mr. Lutnick [Member] | Retirement Plan [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
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		$ 105,000
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] | LPUs [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					7,000,000.0
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] | Revolving Credit Facility [Member]
Related Party Transactions [Abstract]
Loans to inter company									$ 150,000,000.0
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]
Stockholders 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]
Stockholders received on per share					0.463895	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					0.9362
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%
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																			$ 200,000		$ 1,400,000	$ 500,000	$ 400,000
Recognized related party revenues																			8,200,000		6,800,000	21,200,000	19,100,000
Service charges																			16,300,000		13,400,000	44,100,000	39,100,000
Compensation to leased employees																			9,300,000		7,800,000	27,600,000	23,600,000
Repurchase agreements																			1,900,000			1,900,000		$ 1,000,000.0
Recognition of share of foreign exchange gains (losses)																			(500,000)		(1,500,000)	400,000	(3,100,000)
Commissions																			24,000		59,000	171,000	194,000
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
Securities loaned																								15,100,000
Founding partner units remaining | shares																			1,733,418			1,733,418
Cantor [Member] | Revolving Credit Facility [Member]
Related Party Transactions [Abstract]
Principal amount of debt component									250,000,000.0																			$ 400,000,000.0
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									150,000,000.0										$ 0		80,000,000.0	$ 0	80,000,000.0
Interest expense																					1,400,000		3,900,000
Interest expense related discontinued operation																					1,300,000		3,500,000
Interest income from affiliate																			0
Borrowings under credit facility									$ 150,000,000.0
Cantor [Member] | Lucera [Member]
Related Party Transactions [Abstract]
Recognized related party revenues																			200,000		$ 200,000	500,000	$ 600,000
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
CF & Co [Member]
Related Party Transactions [Abstract]
Payment percentage of the gross proceeds from the sale of shares												5.50%
Repurchase agreements																			$ 1,900,000			$ 1,900,000		1,000,000.0
Issuance of Class A common stock (net of costs), shares | shares																			0		2,000,000.0	900,000	32,600,000
Aggregate proceeds from issuance of common stock																					$ 23,100,000	$ 5,300,000	$ 434,900,000
Weighted-average price per share | $ / shares																					$ 11.48	$ 6.17	$ 13.34
Related party expenses																			$ 0		$ 500,000	$ 100,000	$ 8,700,000
Securities loaned																			13,000,000.0			13,000,000.0		$ 15,100,000
Stated interest rate																								2.94%
Guarantee fees																			31,000		31,000	94,000	94,000
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			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] | Securities Financing Transaction, Fair Value [Member]
Related Party Transactions [Abstract]
Securities loaned																			$ 12,900,000			$ 12,900,000		$ 15,300,000
CF & Co [Member] | Maximum [Member]
Related Party Transactions [Abstract]
Stated interest rate																			3.00%			3.00%
CF & Co [Member] | Minimum [Member]
Related Party Transactions [Abstract]
Stated interest rate																			2.65%			2.65%
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 Freedom International Brokerage [Member]
Related Party Transactions [Abstract]
Receivables from Freedom International Brokerage																			$ 1,400,000			$ 1,400,000		1,400,000
Receivables from Cantor related to open derivative contracts																			3,300,000			3,300,000		1,600,000
Payables to Cantor related to open derivative contracts																			2,400,000			2,400,000		2,100,000
Payables to Cantor related to pending equity trades																			25,500,000			25,500,000		0
Receivables from Cantor related to pending trades																								0
Employee Loans [Member]
Related Party Transactions [Abstract]
Employee loans																			266,900,000			266,900,000		216,900,000
Compensation expense (benefit) related to employee loans																			9,300,000		(2,500,000)	23,300,000	3,000,000.0
Interest income																			1,100,000		$ 900,000	3,100,000	$ 2,500,000
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
Castle Oak | 3.750% Senior Notes Due October 1, 2024 [Member] | Underwriting Fees [Member]
Related Party Transactions [Abstract]
Related party expenses	$ 36,000
Cantor Fitzgerald Relief Fund [Member] | Accounts Payable, Accrued and Other Liabilities [Member]
Related Party Transactions [Abstract]
Charitable contributions																			$ 10,100,000			$ 10,100,000		$ 20,000,000.0
Cantor Fitzgerald Relief Fund [Member] | Other Expenses [Member]
Related Party Transactions [Abstract]
Charitable contributions																									$ 40,000,000.0