Schedule I - Parent Company Only Financial Statements - Statements of Operations (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues:
Revenues		$ 1,904,998	$ 2,218,132	$ 2,047,579
Expenses:
Total operating expenses		1,736,290	2,052,850	1,842,282
Income before income taxes and noncontrolling interests		146,270	214,148	282,385
Equity income of subsidiaries		(11,562)	7,250	2,750
Provision for income taxes		36,993	52,436	90,487
Net income available to common stockholders		80,060	117,305	106,732
Basic earnings per share
Net income available to common stockholders	[1]	$ 70,281	$ 104,406	$ 101,641
Basic earnings per share (in usd per share)		$ 0.39	$ 0.59	$ 0.65
Basic weighted-average shares of common stock outstanding (in shares)		179,106	177,774	157,256
Fully diluted earnings per share
Net income for fully diluted shares		$ 70,281	$ 108,160	$ 105,571
Fully diluted earnings per share (in usd per share)		$ 0.39	$ 0.58	$ 0.64
Fully diluted weighted-average shares of common stock outstanding (in shares)		179,690	185,016	163,810
Line of Credit | Revolving Credit Facility
Expenses:
Interest expense		$ 7,984	$ 3,057	$ 118
6.125% Senior Notes | Senior Notes
Expenses:
Interest expense		36,082	36,577	5,545
NEWMARK Group Inc Parent
Revenues:
Interest income		44,067	39,634	27,249
Revenues		44,067	39,634	27,249
Expenses:
Professional and consulting fees		323	763	277
Interest expense		44,067	39,634	27,249
Other expenses		950	930	344
Total operating expenses		45,340	41,327	27,870
Income before income taxes and noncontrolling interests		(1,273)	(1,693)	(621)
Equity income of subsidiaries		113,732	168,358	190,826
Provision for income taxes		32,399	49,360	83,473
Net income available to common stockholders		80,060	117,305	106,732
Basic earnings per share
Net income available to common stockholders		$ 70,281	$ 104,406	$ 101,641
Basic earnings per share (in usd per share)		$ 0.39	$ 0.59	$ 0.65
Basic weighted-average shares of common stock outstanding (in shares)		179,106	177,774	157,256
Fully diluted earnings per share
Net income for fully diluted shares		$ 70,281	$ 108,160	$ 105,571
Fully diluted earnings per share (in usd per share)		$ 0.39	$ 0.58	$ 0.64
Fully diluted weighted-average shares of common stock outstanding (in shares)		179,690	185,016	163,810
Reduction for dividends on preferred stock or units		$ 9,800	$ 12,900	$ 5,100
NEWMARK Group Inc Parent | Line of Credit | Revolving Credit Facility
Expenses:
Interest expense		7,984	3,057	118
NEWMARK Group Inc Parent | 6.125% Senior Notes | Senior Notes
Expenses:
Interest expense		$ 36,082	$ 36,577	$ 5,545

[1] Includes a reduction for dividends on preferred stock or exchangeable preferred partnership units in the amount of $9.8 million, $12.9 million and $5.1 million for the years ended December 31, 2020, 2019 and 2018, respectively.