Related Party Transactions - Additional Information (Detail) - USD ($)																				1 Months Ended					3 Months Ended							6 Months Ended		12 Months Ended
	Jun. 30, 2017	Apr. 21, 2017	Dec. 14, 2016	Nov. 16, 2016	Nov. 11, 2016	Nov. 10, 2016	Nov. 07, 2016	Nov. 04, 2016	Oct. 25, 2016	Sep. 30, 2016	Aug. 16, 2016	May 27, 2016	Mar. 09, 2016	Feb. 28, 2016	Feb. 24, 2016	Feb. 23, 2016	Feb. 09, 2016	Jun. 05, 2015	Oct. 29, 2013	Jun. 30, 2017	May 31, 2017	Apr. 30, 2017	Feb. 28, 2017	Jan. 31, 2017	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Apr. 12, 2017	Aug. 04, 2016	Jul. 31, 2016	Oct. 27, 2015	Jun. 23, 2015	Aug. 31, 2013
Related Party Transactions [Abstract]
Recognized related party revenues																												$ 5,576,000			$ 4,865,000	$ 12,141,000	$ 11,935,000
Commissions																												580,033,000			498,588,000	1,127,159,000	973,675,000
Short-term borrowings	$ 150,000,000																			$ 150,000,000								150,000,000				150,000,000
Interest expense																												16,676,000			14,624,000	31,497,000	28,082,000
Principal sum of loan due from related parties	8,970,000																			8,970,000								8,970,000		$ 6,967,000		8,970,000		$ 6,967,000
Reverse purchase agreement outstanding	0																			0								0		54,659,000		0		54,659,000
Payables to Cantor related to open derivative contracts	39,349,000																			39,349,000								39,349,000		28,976,000		39,349,000		28,976,000
Employee loans	299,595,000																			$ 299,595,000								299,595,000		267,527,000		299,595,000		267,527,000
Interest income																												6,001,000			3,778,000	$ 9,304,000	6,162,000
Repurchase of Class A common stock, shares																				27,634	121,023	248,882							552,955			950,494
Average price per share of common stock repurchased																				$ 12.21	$ 11.57	$ 10.99							$ 11.07			$ 11.14
Aggregate proceeds from issuance of common stock																																	14,961,000
Securities loaned	95,327,000																			$ 95,327,000								95,327,000		0		$ 95,327,000		0
Related party service fee																			10.00%
Professional and consulting fees																												20,736,000			14,336,000	40,316,000	30,054,000
Payments of referrals for leasing or other services																												5,404,000			$ 3,642,000	$ 11,781,000	$ 9,967,000
Limited partnership units exchange ratio																																1
Redemption of limited partnership units															0
Employee stock option exercised, shares																																0
Employee stock option exercise price per share																																$ 0
Repurchases value of shares	297,378,000																			297,378,000								$ 297,378,000		$ 288,743,000		$ 297,378,000		$ 288,743,000
Share Price																								$ 10.23
Redemptions, Number of Units																												1,908,867	2,927,758			4,800,000
Average price for redemption of limited partnership units																												$ 11.20			$ 8.93	$ 10.97	$ 8.85
Loan receivables from related parties	$ 150,000,000																			$ 150,000,000								$ 150,000,000				$ 150,000,000
Partnership Units [Member]
Related Party Transactions [Abstract]
Repurchase of Class A common stock, shares							110,000
Percentage of units proportion							55.00%
Treasury Stock, Retired, Cost Method, Amount							$ 952,600
PPSUs [Member]
Related Party Transactions [Abstract]
Repurchase of Class A common stock, shares							90,000
Average price per share of common stock repurchased							$ 8.60
Percentage of units proportion							45.00%
Treasury Stock, Retired, Cost Method, Amount							$ 773,599
New Class Of Partnership Units [Member]
Related Party Transactions [Abstract]
Gross Revenues Required For Compensation Approval																								$ 5,000,000
5.125% Senior Notes [Member]
Related Party Transactions [Abstract]
Principal amount of debt component												$ 300,000,000
Stated interest rate	5.125%											5.125%								5.125%								5.125%		5.125%		5.125%		5.125%
Mr. Lutnick [Member]
Related Party Transactions [Abstract]
Grant of new class of partnership units															1,500,000
Partnership units non-exchangeable shares granted														2,127,648
Shares held by executive officers														235,357
Non exchangeable partnership units granted																								325,000
New Class Of Partnership Units Surrendered																								325,000
New Partnership Units																																		2,375,000
Non-exchangeable Partnership Unit																								1,710,000
Non-exchangeable Partnership Unit																								665,000
Non exchangeable partnership units granted																								1,661,600
New Class Of Partnership Units Surrendered																								1,661,600
Aggregate Non Exchangeable Partnership Units Surrendered																								1,986,600
Mr. Lutnick [Member] | Partnership Units [Member]
Related Party Transactions [Abstract]
Partnership units non-exchangeable shares granted															750,000
Mr. Lutnick [Member] | PPSUs [Member]
Related Party Transactions [Abstract]
Partnership units non-exchangeable shares granted															291,667
Mr. Lynn [Member]
Related Party Transactions [Abstract]
Average price per share of common stock repurchased																								$ 11.07
Grant of new class of partnership units															2,000,000
Shares held by executive officers																								167,654
Determination price																								$ 6.51
Non exchangeable partnership units exchanged to limited partner unit			250,000
New Partnership Units																																		750,000
Non-exchangeable Partnership Unit																								540,000
Non-exchangeable Partnership Unit																								210,000
Non-exchangeable LPU's redeemed, value																								$ 1,958,641
Non-exchangeable PLPUs redeemed, value																								$ 455,993
Mr. Lynn [Member] | Limited Partnership Units [Member]
Related Party Transactions [Abstract]
Partnership units non-exchangeable shares granted															621,429
Non exchangeable partnership units granted			3,500,000
Redemptions, Number of Units																								0
Mr. Lynn [Member] | PLPUs [Member]
Related Party Transactions [Abstract]
Partnership units non-exchangeable shares granted															241,667
Redemptions, Number of Units																								0
Mr. Lynn [Member] | New Class Of Partnership Units [Member]
Related Party Transactions [Abstract]
Non exchangeable partnership units granted			1,000,000
Mr. Lynn [Member] | Redeemed Price One [Member]
Related Party Transactions [Abstract]
Redemption of limited partnership units																								25,156
Mr. Lynn [Member] | Redeemed Price Two [Member]
Related Party Transactions [Abstract]
Redemption of limited partnership units																								180,115
Mr. Lynn [Member] | Preferred Limited Partnership Units Determination Price Five [Member]
Related Party Transactions [Abstract]
Non-exchangeable PLPUs redeemed																								70,045
Mr. Lynn [Member] | Redeemed Price Three [Member]
Related Party Transactions [Abstract]
Redemption of limited partnership units																								9,480
Mr. Lynn [Member] | Preferred Limited Partnership Units Determination Price Six [Member]
Related Party Transactions [Abstract]
Non-exchangeable PLPUs redeemed																								3,687
Mr. Merkel [Member]
Related Party Transactions [Abstract]
Grant of new class of partnership units															1,000,000
Non exchangeable partnership units granted							200,000
Target revenue to be achieved to grant non exchangeable partnership units																													$ 25,000,000	$ 25,000,000
New Class Of Partnership Units Surrendered							1,714,826
Mr. Merkel [Member] | Scenario, Forecast [Member]
Related Party Transactions [Abstract]
Non exchangeable partnership units granted																									1,000,000
Target revenue to be achieved to grant non exchangeable partnership units																									$ 25,000,000	$ 25,000,000	$ 25,000,000
Mr. Merkel [Member] | Partnership Units [Member]
Related Party Transactions [Abstract]
Partnership units non-exchangeable shares granted															114,583
Redemption of limited partnership units																							110,000
Exchange Right Issued							110,000
Share Granted For Exchange Rights																							110,000
Share redemption amount																							$ 1,216,911
Mr. Merkel [Member] | PPSUs [Member]
Related Party Transactions [Abstract]
Partnership units non-exchangeable shares granted															93,750
Redemption of limited partnership units																							90,000
Exchange Right Issued							90,000
Share Granted For Exchange Rights																							90,000
Share redemption amount																							$ 847,033
Determination price																							$ 9.41
Mr. Windeatt [Member]
Related Party Transactions [Abstract]
Grant of new class of partnership units															75,000									400,000
Redemption of limited partnership units																								46,469
New Partnership Units																																		18,750
Non-exchangeable Partnership Unit																								13,500
Non-exchangeable Partnership Unit																								5,250
Limited Partnership Unites Granted																								100,000
New Partnership Units Exchanged To Limited Partnership Unit																								100,000
Redemptions, Number of Units																								0
Non-exchangeable LPU's redeemed, value																								$ 505,322
Non-exchangeable PLPUs redeemed																								14,866
Non-exchangeable PLPUs redeemed, value																								$ 96,778
Average price for redemption of limited partnership units																								$ 10.87
Non-exchangeable PLPUs redeemed at determination price																								3,206
Non-exchangeable PLPUs redeemed at determination price, value																								$ 25,103
Mr. Windeatt [Member] | Limited Partnership Units [Member]
Related Party Transactions [Abstract]
Partnership units non-exchangeable shares granted															105,188
Mr. Windeatt [Member] | PLPUs [Member]
Related Party Transactions [Abstract]
Partnership units non-exchangeable shares granted															40,906
Mr. Windeatt [Member] | Preferred Limited Partnership Units Determination Price One [Member]
Related Party Transactions [Abstract]
Determination price																								$ 6.51
Non-exchangeable PLPUs redeemed																								782
Mr. Windeatt [Member] | Preferred Limited Partnership Units Determination Price Two [Member]
Related Party Transactions [Abstract]
Determination price																								$ 7.83
Non-exchangeable PLPUs redeemed																								347
Mr. Windeatt [Member] | Limited Partnership [Member]
Related Party Transactions [Abstract]
Redemption of limited partnership units																								2,902
Partnership N [Member]
Related Party Transactions [Abstract]
Conversion percentage of new partnership units															25.00%
Mr. Sadler [Member] | Limited Partnership Units [Member]
Related Party Transactions [Abstract]
Partnership units non-exchangeable shares granted															55,688
Mr. Sadler [Member] | PLPUs [Member]
Related Party Transactions [Abstract]
Partnership units non-exchangeable shares granted															21,656
Minimum [Member]
Related Party Transactions [Abstract]
Minimum gross revenue required			$ 5,000,000
Aqua [Member]
Related Party Transactions [Abstract]
Equity investment																												$ 400,000			$ 800,000	$ 600,000	$ 1,100,000
Principal sum of loan due from related parties	$ 980,000																			$ 980,000								$ 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
Investment ownership percentage amount	49.00%																			49.00%								49.00%				49.00%
Maturity date of subordinated loan																																Sep. 01, 2018
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%								6.00%				6.00%
Class B Units | Lucera [Member]
Related Party Transactions [Abstract]
Purchase of LFI units									9,000
Percentage of outstanding common stock owned by parent								100.00%
Class A Common Stock [Member]
Related Party Transactions [Abstract]
Convertible Notes, shares																																		6,909
Repurchase of Class A common stock, shares																												397,539			797,189	950,494	7,984,235	10,823,942
Average price per share of common stock repurchased															$ 8.40													$ 11.25			$ 9.04	$ 11.14	$ 8.75
Issuance of Class A common stock (net of costs), shares																																33,616		2,004,533
Common stock, shares issued	299,722,000																			299,722,000								299,722,000		292,549,000		299,722,000		292,549,000
Cantor purchase of Cantor units from BGC Holdings upon redemption of founding/working partner units and subsequent repurchases by BGC Holdings, units																																		624,762
Shares held by executive officers															612,958
Class A Common Stock [Member] | Mr. Lutnick [Member]
Related Party Transactions [Abstract]
Repurchase of Class A common stock, shares											36,405
Average price per share of common stock repurchased											$ 8.77
Employee stock option exercised, shares					800,000								250,000
Employee stock option exercise price per share					$ 8.80								$ 8.42
Number of shares issued as a result of stock option exercise					51,064								17,403
Repurchases value of shares																																						$ 350,000
Class A Common Stock [Member] | Mr. Lynn [Member]
Related Party Transactions [Abstract]
Shares held by executive officers															431,782
Partnership units received by related parties															120,000
Class A Common Stock [Member] | Mr. Merkel [Member]
Related Party Transactions [Abstract]
Repurchase of Class A common stock, shares										16,634					4,621
Average price per share of common stock repurchased										$ 8.75
Shares held by executive officers															150,382
Class A Common Stock [Member] | Mr. Merkel [Member] | Partnership Units [Member]
Related Party Transactions [Abstract]
Percentage difference between purchase price of partnership unit and average price of common stock																							2.00%
Class A Common Stock [Member] | Mr. Sadler [Member]
Related Party Transactions [Abstract]
Shares held by executive officers															30,794
Partnership units received by related parties															64,787
Class A Common Stock [Member] | Trusts For Benefit of Merkel's Immediate Family [Member]
Related Party Transactions [Abstract]
Repurchase of Class A common stock, shares										4,131
Class A Common Stock [Member] | Mr. Dalton [Member]
Related Party Transactions [Abstract]
Employee stock option exercised, shares						7,534
Employee stock option exercise price per share						$ 8.87
Class B Common Stock [Member]
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs), shares																												0			0	0	0
Common stock, shares issued	34,848,000																			34,848,000								34,848,000		34,848,000		34,848,000		34,848,000
Cantor and ELX [Member]
Related Party Transactions [Abstract]
Cantor's share of net profit in Tower Bridge																												$ 600,000			$ 100,000	$ 18,000	$ 500,000
April 2017 Sales Agreement | Class A Common Stock [Member]
Related Party Transactions [Abstract]
Common stock, shares issued	0																			0								0				0				0
Cantor [Member]
Related Party Transactions [Abstract]
Mark-up cost of services																																7.50%
Company ownership of Tower Bridge																																52.00%
Cantor ownership of Tower Bridge	48.00%																			48.00%								48.00%				48.00%
Recognition of share of foreign exchange gains (losses)																												$ (2,300,000)			(1,500,000)	$ (2,800,000)	(1,400,000)
Commissions																												51,000			28,000	131,000	80,000
Investment in asset-backed commercial paper program	$ 0																			$ 0								0		$ 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
Current interest rate		4.47%
Credit agreement maturity date		Apr. 20, 2018
Interest income from affiliate																												1,300,000				1,300,000
Reverse purchase agreement outstanding																														54,700,000				54,700,000
Professional and consulting fees																												$ 0			0	$ 0	0
Founding partner units remaining	991,182																			991,182								991,182				991,182
Cantor [Member] | Audit Committee [Member]
Related Party Transactions [Abstract]
Development expenses																												$ 1,200,000
Cantor [Member] | Revolving Credit Facility [Member]
Related Party Transactions [Abstract]
Debt Instrument, description																																The outstanding balances would bear interest at the higher of the borrower’s or the lender’s short-term borrowing rate then in effect, plus 1%.
Cantor [Member] | Maximum [Member] | Audit Committee [Member]
Related Party Transactions [Abstract]
Development expenses																	$ 1,400,000
Cantor [Member] | Maximum [Member] | Revolving Credit Facility [Member]
Related Party Transactions [Abstract]
Principal amount of debt component																																								$ 150,000,000
Cantor [Member] | Lucera [Member]
Related Party Transactions [Abstract]
Recognized related party revenues																												$ 300,000			900,000	$ 700,000	1,800,000
Cantor [Member] | Aqua [Member]
Related Party Transactions [Abstract]
Investment ownership percentage	51.00%																			51.00%								51.00%				51.00%
Cantor [Member] | Class B Units | Lucera [Member]
Related Party Transactions [Abstract]
Equity investment								$ 24,200,000
Post-closing adjustment								$ 4,800,000
Cantor [Member] | Class A Common Stock [Member]
Related Party Transactions [Abstract]
Repurchase of Class A common stock, shares																5,000,000
Average price per share of common stock repurchased																$ 8.72
Cantor and ELX [Member]
Related Party Transactions [Abstract]
Recognized related party revenues																												$ 5,600,000			4,900,000	$ 12,100,000	11,900,000
Service charges																												13,600,000			9,800,000	28,700,000	22,900,000
Compensation to leased employees																												8,200,000			6,200,000	$ 16,900,000	12,900,000
CF Group Management, Inc. [Member]
Related Party Transactions [Abstract]
Number of shares, right to exchange from Class A to Class B common stock																		34,649,693
Cantor Rights to Purchase Exchangeable Units [Member]
Related Party Transactions [Abstract]
Cantor purchase of Cantor units from BGC Holdings upon redemption of founding/working partner units and subsequent repurchases by BGC Holdings, units							624,762
Aggregate exchangeable limited partnership units purchased							141,523
Aggregate consideration amount							$ 560,190
Number of non-exchangeable founding/working partner units redeemed							141,523
Aggregate consideration of non-exchangeable founding/working partner units redeemed							$ 560,190
Founding partner units remaining							548,259
Cantor Rights to Purchase Exchangeable Units [Member] | Exchange Rights [Member]
Related Party Transactions [Abstract]
Aggregate exchangeable limited partnership units purchased							483,239
Aggregate consideration amount							$ 1,796,367
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																																34,649,693
Cantor and CFGM [Member] | Class A Common Stock [Member]
Related Party Transactions [Abstract]
Convertible Notes, shares	14,676,743
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																																34,649,693
Cantor and Freedom International Brokerage [Member]
Related Party Transactions [Abstract]
Receivables from Freedom International Brokerage	$ 1,900,000																			$ 1,900,000								1,900,000		1,300,000		$ 1,900,000		1,300,000
Receivables from Cantor related to open derivative contracts	2,800,000																			2,800,000								2,800,000		3,000,000		2,800,000		3,000,000
Payables to Cantor related to open derivative contracts	3,900,000																			3,900,000								3,900,000		1,000,000		3,900,000		1,000,000
Receivables from Cantor related to pending equity trades	2,300,000																			2,300,000								2,300,000		0		2,300,000		0
Employee Loans [Member]
Related Party Transactions [Abstract]
Employee loans	299,600,000																			299,600,000								299,600,000		267,500,000		299,600,000		267,500,000
Compensation expense related to employee loans																												8,700,000			10,600,000	15,900,000	21,100,000
Interest income																												1,600,000			1,700,000	3,100,000	2,700,000
CF & Co [Member]
Related Party Transactions [Abstract]
Securities loaned	95,300,000																			95,300,000								95,300,000		0		95,300,000		0
Guarantee fees																												31,300			$ 31,300	62,500	$ 62,500
CF & Co [Member] | 5.125% Senior Notes [Member] | Underwriting Fees
Related Party Transactions [Abstract]
Underwriting fees												$ 500,000
Purchased senior notes	15,000,000																			15,000,000								15,000,000				15,000,000
CF & Co [Member] | Securities Financing Transaction, Fair Value
Related Party Transactions [Abstract]
Securities loaned	$ 95,400,000																			$ 95,400,000								$ 95,400,000				$ 95,400,000
CF & Co [Member] | Maximum [Member]
Related Party Transactions [Abstract]
Stated interest rate	2.00%																			2.00%								2.00%			2.00%	2.00%	2.00%
CF & Co [Member] | Minimum [Member]
Related Party Transactions [Abstract]
Stated interest rate	1.30%																			1.30%								1.30%			1.30%	1.30%	1.30%
CF & Co [Member] | November 2014 Sales Agreement [Member]
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs), shares																												1,500,000			3,300,000	4,300,000	3,700,000
Aggregate proceeds from issuance of common stock																												$ 17,100,000			$ 30,300,000	$ 47,800,000	$ 33,600,000
Weighted-average price per share																												$ 11.42			$ 9.18	$ 11.18	$ 9.16
Related party expenses																												$ 300,000			$ 600,000	$ 1,000,000	$ 700,000
Castle Oak | 5.125% Senior Notes [Member] | Underwriting Fees
Related Party Transactions [Abstract]
Underwriting fees												$ 18,000
CCRE [Member]
Related Party Transactions [Abstract]
Recognized related party revenues																												2,900,000			900,000	4,300,000	1,800,000
Payments of referrals for leasing or other services																												0			$ 100,000	$ 0	$ 100,000
Chief Financial Officer [Member]
Related Party Transactions [Abstract]
Redemption of limited partnership units																						17,115
Determination price																						$ 11.07
Non-exchangeable LPU's redeemed, value																						$ 188,634
Non-exchangeable PLPUs redeemed, value																						$ 73,682
Chief Financial Officer [Member] | Limited Partnership Units [Member]
Related Party Transactions [Abstract]
Redemptions, Number of Units																						1,849
Chief Financial Officer [Member] | PLPUs [Member]
Related Party Transactions [Abstract]
Redemptions, Number of Units																						0
Chief Financial Officer [Member] | Preferred Limited Partnership Units Determination Price Three [Member]
Related Party Transactions [Abstract]
Non-exchangeable PLPUs redeemed																						6,656
Chief Financial Officer [Member] | Preferred Limited Partnership Units Determination Price Four [Member]
Related Party Transactions [Abstract]
Non-exchangeable PLPUs redeemed																						719
Relief Fund [Member]
Related Party Transactions [Abstract]
Loan receivables from related parties																																			$ 1,000,000
Promissory note from related party																																					$ 1,000,000
Promissory note repayment date																																Aug. 04, 2016
Relief Fund [Member] | Accounts Payable, Accrued and Other Liabilities [Member]
Related Party Transactions [Abstract]
Charitable contributions	$ 31,200,000																			$ 31,200,000								$ 31,200,000		$ 30,700,000		$ 31,200,000		$ 30,700,000
Relief Fund [Member] | Other Expenses [Member]
Related Party Transactions [Abstract]
Charitable contributions																																			$ 40,000,000
Cantor Fitzgerald Relief Fund [Member] | Class A Common Stock [Member]
Related Party Transactions [Abstract]
Repurchase of Class A common stock, shares				166,238												970,639
Average price per share of common stock repurchased				$ 9.74												$ 8.72