Related Party Transactions - Additional Information (Detail) - USD ($)																					1 Months Ended				3 Months Ended						12 Months Ended
	Mar. 31, 2017	Dec. 14, 2016	Nov. 16, 2016	Nov. 11, 2016	Nov. 10, 2016	Nov. 07, 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	Jan. 15, 2016	Jun. 05, 2015	Apr. 13, 2015	Jun. 03, 2014	Oct. 29, 2013	Apr. 30, 2017	Mar. 31, 2017	Feb. 28, 2017	Jan. 31, 2017	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Apr. 21, 2017	Nov. 04, 2016	Aug. 04, 2016	Jul. 31, 2016	Oct. 27, 2015	Oct. 01, 2015	Jun. 23, 2015	Dec. 09, 2014	Aug. 31, 2013
Related Party Transactions [Abstract]
Recognized related party revenues																												$ 6,565,000		$ 7,070,000
Equity investment																												521,000		328,000
Commissions																												547,126,000		475,087,000
Interest expense																												14,821,000		13,458,000
Reverse purchase agreement outstanding																													$ 54,659,000		$ 54,659,000
Payables to Cantor related to open derivative contracts	$ 18,297,000																					$ 18,297,000						18,297,000	28,976,000		28,976,000
Employee loans	278,959,000																					$ 278,959,000						278,959,000	267,527,000		267,527,000
Interest income																												$ 3,303,000		2,384,000
Repurchase of Class A common stock, shares																						27,039	98,660	427,256				552,955
Average price per share of common stock repurchased																						$ 11.20	$ 11.29	$ 11.01				$ 11.07
Securities loaned	$ 87,293,000																					$ 87,293,000						$ 87,293,000
Related party service fee																				10.00%
Professional and consulting fees																												19,580,000		15,718,000
Payments of referrals for leasing or other services																												$ 6,377,000		$ 6,325,000
Limited partnership units exchange ratio																												1
Founding partner units remaining	997,773																					997,773						997,773
Redemption of limited partnership units													0
Employee stock option exercised, shares																												0
Employee stock option exercise price per share																												$ 0
Repurchases value of shares	$ 293,555,000																					$ 293,555,000						$ 293,555,000	288,743,000		288,743,000
Share Price																								10.23
Repurchases, Average Price Paid per Share																						$ 11.20	$ 11.29	$ 11.01				$ 11.07
Redemptions, Number of Units																												2,927,758
Average price for redemption of limited partnership units																												$ 10.85		$ 8.65
Principal sum of loan due from related parties	$ 8,166,000																					$ 8,166,000						$ 8,166,000	$ 6,967,000		$ 6,967,000
Fees from related parties																												6,565,000		$ 7,070,000
Related party capital contributions																												$ 0
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
Repurchases, Average Price Paid per Share						$ 8.60
New Class Of Partnership Units
Related Party Transactions [Abstract]
Gross Revenues Required For Compensation Approval																								$ 5,000,000
5.375% Senior Notes [Member]
Related Party Transactions [Abstract]
Principal amount of debt component																																									$ 300,000,000
Stated interest rate	5.375%																					5.375%						5.375%	5.375%		5.375%										5.375%
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%
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
Repurchases, Average Price Paid per Share																								$ 11.07
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
Related Party Transactions [Abstract]
Non exchangeable partnership units granted		1,000,000
Mr. Lynn [Member] | Redeemed Price One
Related Party Transactions [Abstract]
Redemption of limited partnership units																								25,156
Mr. Lynn [Member] | Redeemed Price Two
Related Party Transactions [Abstract]
Redemption of limited partnership units																								180,115
Mr. Lynn [Member] | Preferred Limited Partnership Units Determination Price Five
Related Party Transactions [Abstract]
Non-exchangeable PLPUs redeemed																								70,045
Mr. Lynn [Member] | Redeemed Price Three
Related Party Transactions [Abstract]
Redemption of limited partnership units																								9,480
Mr. Lynn [Member] | Preferred Limited Partnership Units Determination Price Six
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
Related Party Transactions [Abstract]
Determination price																								$ 6.51
Non-exchangeable PLPUs redeemed																								782
Mr. Windeatt [Member] | Preferred Limited Partnership Units Determination Price Two
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]
Investment ownership percentage amount	49.00%																					49.00%						49.00%
Equity investment																												$ 200,000		$ 300,000
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
Principal sum of loan due from related parties	$ 980,000																					$ 980,000						$ 980,000
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%
Class B Units [Member] | Lucera [Member]
Related Party Transactions [Abstract]
Purchase of LFI units							9,000												1,000
Investment ownership percentage amount																20.00%			10.00%
Investment in equity amount																			$ 6,500,000
Option to purchase additional units																			1,000
Additional investment in investment in equity																			$ 6,500,000
Equity investment																$ 6,500,000
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																												552,955		7,187,046	10,823,942
Average price per share of common stock repurchased													$ 8.40															$ 11.07		$ 8.72
Issuance of Class A common stock (net of costs), shares																												9,942			2,004,533
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
Repurchases, Average Price Paid per Share													$ 8.40															$ 11.07		$ 8.72
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
Repurchases, Average Price Paid per Share									$ 8.77
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
Repurchases, Average Price Paid per Share								$ 8.75
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
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%
Recognition of share of foreign exchange gains (losses)																												$ (510,500)		$ 149,000
Commissions																												80,000		52,000
Reverse purchase agreement outstanding	$ 4,900,000																					$ 4,900,000						4,900,000	54,700,000		$ 54,700,000
Professional and consulting fees																												0		0
Investment in asset-backed commercial paper program	0																					0						0	0		0
Investment authorization in asset-backed commercial paper program																																										$ 350,000,000
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%.
Borrowings outstanding under the Credit Agreement	$ 0																					$ 0						$ 0
Line of credit facility, borrowing amount																																							$ 100,000,000
Interest expense																												0		0
Cantor [Member] | Revolving Credit Facility [Member] | Subsequent Event
Related Party Transactions [Abstract]
Line of credit facility, borrowing amount																																		$ 150,000,000
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																												400,000		800,000
Fees from related parties																												$ 400,000		800,000
Cantor [Member] | Aqua [Member]
Related Party Transactions [Abstract]
Investment ownership percentage	51.00%																					51.00%						51.00%
Cantor [Member] | Class B Units [Member] | 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
Repurchases, Average Price Paid per Share														$ 8.72
Cantor and ELX [Member]
Related Party Transactions [Abstract]
Recognized related party revenues																												6,600,000		7,100,000
Service charges																												15,100,000		12,900,000
Compensation to leased employees																												8,700,000		6,700,000
Cantor's share of net profit in Tower Bridge																												(600,000)		500,000
Fees from related parties																												$ 6,600,000		7,100,000
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	14,683,401																	24,042,599
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	$ 2,100,000																					$ 2,100,000						$ 2,100,000	1,300,000		1,300,000
Receivables from Cantor related to open derivative contracts	3,900,000																					3,900,000						3,900,000	3,000,000		3,000,000
Payables to Cantor related to open derivative contracts	5,800,000																					5,800,000						5,800,000	1,000,000		1,000,000
Payables to Cantor related to pending equity trades	3,900,000																					3,900,000						3,900,000	0		0
Employee Loans [Member]
Related Party Transactions [Abstract]
Employee loans	279,000,000																					279,000,000						279,000,000	267,500,000		267,500,000
Compensation expense related to employee loans																												7,200,000		10,500,000
Interest income																												1,600,000		1,000,000
CF & Co [Member]
Related Party Transactions [Abstract]
Securities loaned	87,300,000																					87,300,000						87,300,000
Guarantee fees																												31,250		31,250
CF & Co [Member] | Underwriting Fees [Member]
Related Party Transactions [Abstract]
Underwriting fees																																	$ 252,000
CF & Co [Member] | 5.125% Senior Notes [Member] | Underwriting Fees [Member]
Related Party Transactions [Abstract]
Underwriting fees										$ 500,000
Purchased senior notes																														$ 15,000,000
CF & Co [Member] | Securities Financing Transaction, Fair Value
Related Party Transactions [Abstract]
Securities loaned	$ 87,400,000																					$ 87,400,000						$ 87,400,000
CF & Co [Member] | November 2014 Sales Agreement [Member]
Related Party Transactions [Abstract]
Issuance of Class A common stock (net of costs), shares																												2,800,000		400,000
Aggregate proceeds from issuance of common stock																												$ 30,700,000		$ 3,400,000
Weighted-average price per share																												$ 11.05		$ 9.01
Related party expenses																												$ 600,000		$ 100,000
CF & Co [Member] | Maximum [Member]
Related Party Transactions [Abstract]
Stated interest rate	1.75%																					1.75%						1.75%
CF & Co [Member] | Minimum [Member]
Related Party Transactions [Abstract]
Stated interest rate	1.41%																					1.41%						1.41%
CastleOak [Member] | Underwriting Fees [Member]
Related Party Transactions [Abstract]
Underwriting fees																																	$ 18,000
CastleOak [Member] | 5.125% Senior Notes [Member] | Underwriting Fees [Member]
Related Party Transactions [Abstract]
Underwriting fees										$ 18,000
CCRE [Member]
Related Party Transactions [Abstract]
Recognized related party revenues																												$ 1,400,000		700,000
Payments of referrals for leasing or other services																												0		0
Fees from related parties																												$ 1,400,000		$ 700,000
Chief Financial Officer | Subsequent Event
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 | Limited Partnership Units [Member] | Subsequent Event
Related Party Transactions [Abstract]
Redemptions, Number of Units																					1,849
Chief Financial Officer | PLPUs [Member] | Subsequent Event
Related Party Transactions [Abstract]
Redemptions, Number of Units																					0
Chief Financial Officer | Preferred Limited Partnership Units Determination Price Three | Subsequent Event
Related Party Transactions [Abstract]
Non-exchangeable PLPUs redeemed																					6,656
Chief Financial Officer | Preferred Limited Partnership Units Determination Price Four | Subsequent Event
Related Party Transactions [Abstract]
Non-exchangeable PLPUs redeemed																					719
Relief Fund [Member]
Related Party Transactions [Abstract]
Interest-free loan to related party																																$ 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	$ 30,700,000																					$ 30,700,000						$ 30,700,000	$ 30,700,000		$ 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
Repurchases, Average Price Paid per Share			$ 9.74											$ 8.72