Organization and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands											1 Months Ended	3 Months Ended		9 Months Ended
	Nov. 30, 2018	Nov. 23, 2018	Sep. 05, 2018	Sep. 04, 2018	Mar. 19, 2018	Mar. 07, 2018	Dec. 26, 2017	Dec. 15, 2017	Sep. 08, 2017	Jun. 28, 2013	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018	Sep. 26, 2018	Jun. 18, 2018	Jan. 01, 2018	Dec. 13, 2017
Description Of Business [Line Items]
Equity method investments											$ 99,966			$ 99,966			$ 101,275
Purchase of units						16,600,000
Purchase value of units						$ 242,000
Proceeds from issuance of exchangeable preferred partnership units															$ 265,569
Number of common shares under spin off transaction	14,800,000
Ownership interest percentage	94.00%
Economic interest ownership percentage	87.00%
Common stock units issued	1,458,931
Exchange Ratio Number of Common Shares Received Under Spin Off Transaction											93.62%			93.62%
Assets											$ 3,583,680			$ 3,583,680			3,454,157
Liabilities											2,575,872			2,575,872			2,371,188
Retained earnings											344,891			344,891			277,952
Noncontrolling interests											368,538			368,538			$ 489,230
ROU asset											202,031			202,031
ROU liability											254,061			254,061
ASU No. 2014-09 [Member] | Impact of ASC 606 [Member]
Description Of Business [Line Items]
Assets																				$ 64,400
Liabilities																				45,600
Retained earnings																				16,500
Noncontrolling interests																				$ 2,300
ASU 2016-02
Description Of Business [Line Items]
ROU asset																$ 178,800
ROU liability																$ 226,700
Newmark OpCo [Member]
Description Of Business [Line Items]
Number of common shares under spin off transaction	7,000,000.0
RBC [Member]
Description Of Business [Line Items]
Proceeds from issuance of exchangeable preferred partnership units												$ 113,200	$ 152,900
RBC [Member] | Newmark OpCo [Member]
Description Of Business [Line Items]
Exchangeable preferred limited partnership units issued											325,000			325,000				$ 150,000	$ 175,000
NASDAQ [Member] | Marketable Securities [Member]
Description Of Business [Line Items]
Fair value of common stock expects to receive											98,600			$ 98,600
NASDAQ [Member] | RBC [Member]
Description Of Business [Line Items]
Reclassified of EPU from noncontrolling interests to payable											$ 93,500
NASDAQ [Member] | Maximum [Member] | Forward Contracts [Member] | RBC [Member]
Description Of Business [Line Items]
Shares received from transaction														992,247
Intercompany Credit Agreement [Member]
Description Of Business [Line Items]
Stated interest rate				6.50%
BGC Partners Inc [Member] | NASDAQ [Member]
Description Of Business [Line Items]
Consideration received on sale of assets										$ 750,000
Period for expected payment under Common stock transaction										15 years
BGC Partners Inc [Member] | NASDAQ [Member] | Maximum [Member]
Description Of Business [Line Items]
Expected payment of shares under common stock transaction										14,883,705
BGC Partners Inc [Member] | NASDAQ [Member] | Minimum [Member]
Description Of Business [Line Items]
Gross revenue on expected payment per year under common stock transaction.										$ 25,000
BGC Partners Inc [Member] | Intercompany Credit Agreement [Member]
Description Of Business [Line Items]
Proceeds from debt					$ 150,000
2042 Promissory Note [Member]
Description Of Business [Line Items]
Stated interest rate																					8.125%
Repayments of debt			$ 112,500	$ 112,500
2019 Promissory Note [Member]
Description Of Business [Line Items]
Stated interest rate																					5.375%
Repayments of debt		$ 300,000
Class A Common Stock [Member]
Description Of Business [Line Items]
Exchange Ratio Number of Common Shares Received Under Spin Off Transaction	46.3895%
Number of common shares under spin off transaction	9,400,000
Common stock, shares issued	131,886,409										158,931,531			158,931,531			156,966,336
Common stock units issued	449,917
Class A Common Stock [Member] | Newmark OpCo [Member]
Description Of Business [Line Items]
Number of common shares under spin off transaction	6,900,000
Class B Common Stock [Member]
Description Of Business [Line Items]
Exchange Ratio Number of Common Shares Received Under Spin Off Transaction	46.3895%
Number of common shares under spin off transaction	5,400,000
Common stock, shares issued	21,285,537										21,285,533			21,285,533			21,285,533
IPO [Member] | Class A Common Stock [Member]
Description Of Business [Line Items]
Stock issued during period, shares, new issues								20,000,000
Closing price per share								$ 14.00
Options to purchase additional shares of common stock period								30 days
Option to purchase additional shares							3,000,000
CF Real Estate Finance Holdings, L.P. [Member]
Description Of Business [Line Items]
Equity method investments									$ 100,000
BPF [Member]
Description Of Business [Line Items]
Business acquisition date									Sep. 08, 2017
Ownership percentage acquired									100.00%