Schedule I - Parent Company Only - Organization and Basis of Presentation (Detail)						12 Months Ended
	Nov. 30, 2018 shares	Nov. 30, 2018 shares	Dec. 26, 2017 USD ($) shares	Dec. 15, 2017 $ / shares shares	Sep. 08, 2017 USD ($)	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Nov. 28, 2018 USD ($)	Sep. 04, 2018 USD ($)	Mar. 19, 2018 USD ($)	Dec. 13, 2017	Nov. 22, 2017 USD ($)
Description Of Business [Line Items]
Equity method investment | $						$ 99,966,000	$ 101,275,000
Proceeds from the Initial Public Offering, net of underwriting discounts | $						$ 0	$ (8,870,000)	$ 304,290,000
Fully diluted weighted-average shares of common stock outstanding (in shares)						185,016,000	163,810,000	138,398,000
Number of units converted to common stock (in shares)		14,800,000
Ownership interest percentage		94.00%
Economic interest ownership percentage		87.00%
Common stock units issued (in shares)	1,458,931	1,458,931
Long-term debt | $						$ 589,294,000	$ 537,926,000
Intercompany Credit Agreement
Description Of Business [Line Items]
Debt instrument face amount | $										$ 112,500,000
Stated interest rate										6.50%
Revolving Credit Facility
Description Of Business [Line Items]
Maximum revolving credit | $													$ 400,000,000.0
Line of Credit | Revolving Credit Facility
Description Of Business [Line Items]
Stated interest rate						3.76%
Maximum revolving credit | $									$ 250,000,000.0
Long-term debt | $						$ 48,917,000	$ 0
Debt issuance costs | $						$ 1,100,000
Class A Common Stock
Description Of Business [Line Items]
Stock issued (in shares)								23,000,000
Number of units converted to common stock (in shares)		9,400,000
Common stock, shares issued (in shares)	131,886,409	131,886,409				160,833,463	156,966,336
Common stock units issued (in shares)	449,917	449,917
Limited partnership units exchange ratio						0.9400						1
Class B Common Stock
Description Of Business [Line Items]
Common stock, shares issued (in 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)				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
CF Real Estate Finance Holdings, L.P.
Description Of Business [Line Items]
Equity method investment | $					$ 100,000,000.0
BGC Partners Inc
Description Of Business [Line Items]
Exchange ratio (per share)	97.93%	97.93%
BGC Partners Inc | Class B Common Stock
Description Of Business [Line Items]
Exchange ratio (per share)	46.3895%	46.3895%
BPF
Description Of Business [Line Items]
Ownership percentage acquired					100.00%
NEWMARK Group Inc Parent
Description Of Business [Line Items]
Proceeds from the Initial Public Offering, net of underwriting discounts | $						$ 0	$ 0	$ 304,290,000
Fully diluted weighted-average shares of common stock outstanding (in shares)						185,016,000	163,810,000	138,398,000
Repayments of debt | $						$ 105,000,000	$ 670,710,000	$ 304,290,000
Long-term debt | $						$ 589,294,000	537,926,000
NEWMARK Group Inc Parent | Line of Credit | Revolving Credit Facility
Description Of Business [Line Items]
Stated interest rate						3.76%
Maximum revolving credit | $									$ 250,000,000.0
Long-term debt | $						$ 48,900,000	$ 0
Debt issuance costs | $						$ 1,100,000
NEWMARK Group Inc Parent | IPO | Class A Common Stock
Description Of Business [Line Items]
Stock issued (in 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
Option to purchase additional shares (in shares)			3,000,000
Proceeds from the Initial Public Offering, net of underwriting discounts | $			$ 295,400,000
NEWMARK Group Inc Parent | CF Real Estate Finance Holdings, L.P.
Description Of Business [Line Items]
Equity method investment | $					$ 100,000,000.0
NEWMARK Group Inc Parent | BGC Partners Inc
Description Of Business [Line Items]
Number of units converted to common stock (in shares)	14,800,000
Ownership interest percentage	94.00%
Economic interest ownership percentage	87.00%
Common stock units issued (in shares)	1,458,931	1,458,931
NEWMARK Group Inc Parent | BGC Partners Inc | Class A Common Stock
Description Of Business [Line Items]
Exchange ratio (per share)	46.3895%	46.3895%
Number of units converted to common stock (in shares)	9,400,000
Common stock, shares issued (in shares)	131,886,409	131,886,409
Common stock units issued (in shares)	449,917	449,917
NEWMARK Group Inc Parent | BGC Partners Inc | Class B Common Stock
Description Of Business [Line Items]
Exchange ratio (per share)	46.3895%	46.3895%
Number of units converted to common stock (in shares)	5,400,000
Common stock, shares issued (in shares)	21,285,537	21,285,537
NEWMARK Group Inc Parent | BPF
Description Of Business [Line Items]
Number of trading days prior to closing					3 days
NEWMARK Group Inc Parent | Berkeley Point Financial
Description Of Business [Line Items]
Ownership percentage acquired					100.00%
Newmark OpCo
Description Of Business [Line Items]
Number of units converted to common stock (in shares)		7,000,000
Newmark OpCo | Class A Common Stock
Description Of Business [Line Items]
Number of units converted to common stock (in shares)		6,900,000
Newmark OpCo | Class B Common Stock
Description Of Business [Line Items]
Number of units converted to common stock (in shares)		5,400,000
Newmark OpCo | BGC Holdings, L.P. | Intercompany Credit Agreement
Description Of Business [Line Items]
Maximum revolving credit | $											$ 150,000,000
Newmark OpCo | BGC Partners Inc
Description Of Business [Line Items]
Number of units converted to common stock (in 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)	6,900,000