Organization and Basis of Presentation (Details)													12 Months Ended
	Jan. 01, 2020 USD ($)	Dec. 02, 2019 USD ($) shares	Nov. 30, 2018 shares	Nov. 30, 2018 shares	Nov. 23, 2018 USD ($)	Sep. 05, 2018 USD ($)	Sep. 04, 2018 USD ($)	Mar. 19, 2018 USD ($)	Mar. 07, 2018 USD ($) $ / shares shares	Dec. 26, 2017 shares	Dec. 15, 2017 USD ($) $ / shares shares	Jun. 28, 2013 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Nov. 30, 2019 USD ($)	Jan. 01, 2019 USD ($)	Sep. 26, 2018 USD ($) tranche contract	Jun. 18, 2018 USD ($)	Jan. 01, 2018 USD ($)	Dec. 13, 2017	Sep. 08, 2017 USD ($)
Description Of Business [Line Items]
Assets													$ 3,201,599,000	$ 3,454,157,000
Equity method investment													99,966,000	101,275,000
Stock issued, value															$ 295,419,000
Proceeds from issuance of EPUs													0	262,169,000	$ 0
Marketable securities													36,795,000	48,942,000		$ 98,600,000
Shares settled (in shares) | shares		898,685
Shares settled fair value		$ 93,500,000
Shares owned (in shares) | shares		93,562
Number of units converted to common stock (in shares) | shares				14,800,000
Ownership interest percentage				94.00%
Economic interest ownership percentage				87.00%
Common stock units issued (in shares) | shares			1,458,931	1,458,931
Liabilities													2,239,457,000	2,371,188,000
Retained earnings													313,112,000	277,952,000
Noncontrolling interests													340,961,000	$ 489,230,000
Right-of-use assets													201,661,000
Operating lease liability													$ 255,126,000
CF Real Estate Finance Holdings, L.P.
Description Of Business [Line Items]
Equity method investment																						$ 100,000,000.0
Class A Common Stock
Description Of Business [Line Items]
Stock issued (in shares) | shares															23,000,000
Limited partnership interest spinoff ratio				46.3895%
Number of units converted to common stock (in shares) | shares				9,400,000
Common stock, shares issued (in shares) | shares			131,886,409	131,886,409									160,833,463	156,966,336
Common stock units issued (in shares) | shares			449,917	449,917
Limited partnership units exchange ratio													0.9400								1
Class B Common Stock
Description Of Business [Line Items]
Limited partnership interest spinoff ratio				46.3895%
Common stock, shares issued (in shares) | shares			21,285,537	21,285,537									21,285,533	21,285,533
IPO | Class A Common Stock
Description Of Business [Line Items]
Stock issued (in shares) | shares											20,000,000
Closing price per share (in dollars per share) | $ / shares											$ 14.00
Options to purchase additional shares of common stock period (in shares)											30 days
Stock issued, value											$ 295,400,000
Over-Allotment Option | Class A Common Stock
Description Of Business [Line Items]
Stock issued (in shares) | shares											3,000,000
Option to purchase additional shares (in shares) | shares										3,000,000
2042 Promissory Note
Description Of Business [Line Items]
Stated interest rate														8.125%							8.125%
Repayments of debt						$ 112,500,000	$ 112,500,000
2019 Promissory Note
Description Of Business [Line Items]
Stated interest rate																					5.375%
Repayments of debt					$ 300,000,000.0
Intercompany Credit Agreement
Description Of Business [Line Items]
Stated interest rate							6.50%
BGC Partners Inc | Intercompany Credit Agreement
Description Of Business [Line Items]
Borrowing of debt								$ 150,000,000.0
BGC Partners LP and its Operating Subsidiaries | Newmark Units
Description Of Business [Line Items]
Closing price per share (in dollars per share) | $ / shares									$ 14.57
Purchase of units | shares									16,600,000
Purchase value of units									$ 242,000,000.0
Limited partnership units exchange ratio													940,000
Accounting Standards Update 2016-02
Description Of Business [Line Items]
Right-of-use assets																	$ 178,800,000
Operating lease liability																	$ 226,700,000
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | ASU No. 2014-09
Description Of Business [Line Items]
Assets																				$ 64,400,000
Liabilities																				45,600,000
Retained earnings																				16,500,000
Noncontrolling interests																				$ 2,300,000
Nasdaq Omx
Description Of Business [Line Items]
Purchase consideration paid in cash												$ 750,000,000.0
Period for expected payment under Common stock transaction												15 years
Maximum | Accounting Standards Update 2016-13 | Forecast
Description Of Business [Line Items]
Impact of adoption new accounting standard	$ 28,000,000.0
Maximum | Nasdaq Omx
Description Of Business [Line Items]
Shares received from transaction (in shares) | shares														992,247,000
Maximum | Nasdaq Omx | BGC Partners Inc
Description Of Business [Line Items]
Expected payment of shares under common stock transaction | shares												14,883,705
Minimum | Accounting Standards Update 2016-13 | Forecast
Description Of Business [Line Items]
Impact of adoption new accounting standard	$ 18,000,000.0
Minimum | Nasdaq Omx | BGC Partners Inc
Description Of Business [Line Items]
Gross revenue on expected payment per year under common stock transaction.												$ 25,000,000.0
RBC
Description Of Business [Line Items]
Proceeds from issuance of EPUs														$ 266,100,000
Newmark OpCo
Description Of Business [Line Items]
Number of units converted to common stock (in shares) | shares				7,000,000
Newmark OpCo | Class A Common Stock
Description Of Business [Line Items]
Number of units converted to common stock (in shares) | shares				6,900,000
Newmark OpCo | Class B Common Stock
Description Of Business [Line Items]
Number of units converted to common stock (in shares) | shares				5,400,000
Newmark OpCo | 2042 Promissory Note
Description Of Business [Line Items]
Stated interest rate																					8.125%
Repayments of debt						$ 112,500,000
Newmark OpCo | 2019 Promissory Note
Description Of Business [Line Items]
Stated interest rate																					5.375%
Repayments of debt					$ 300,000,000.0
Newmark OpCo | BGC Partners Inc
Description Of Business [Line Items]
Number of units converted to common stock (in shares) | shares			7,000,000.0
Newmark OpCo | BGC Partners Inc | Class A Common Stock
Description Of Business [Line Items]
Number of units converted to common stock (in shares) | shares			6,900,000
Newmark OpCo | RBC
Description Of Business [Line Items]
Exchangeable preferred limited partnership units issued (in shares)													$ 325,000,000					$ 150,000,000.0	$ 175,000,000.0
Exchangeable preferred limited partnership units, number of tranches | tranche																		4
Noncontrolling Interests
Description Of Business [Line Items]
Redemption of EPUs													$ 93,480,000
BPF
Description Of Business [Line Items]
Ownership percentage acquired																						100.00%
Nasdaq Forwards
Description Of Business [Line Items]
Number of derivative contracts | contract																		4