Schedule I - Parent Company Only Financial Statements - Statements of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Revenues		$ 2,218,132	$ 2,047,579	$ 1,596,450
Expenses:
Total operating expenses		2,052,850	1,842,282	1,470,589
Income before income taxes and noncontrolling interests		214,148	282,385	202,574
Equity income of subsidiaries		7,250	2,750	1,562
Provision for income taxes		52,436	90,487	57,478
Net income available to common stockholders		117,305	106,732	144,492
Basic earnings per share
Net income available to common stockholders	[1]	$ 104,406	$ 101,641	$ 144,492
Basic earnings per share (in usd per share)		$ 0.59	$ 0.65	$ 1.08
Basic weighted-average shares of common stock outstanding (in shares)		177,774,000	157,256,000	133,413,000
Fully diluted earnings per share
Net income for fully diluted shares		$ 108,160	$ 105,571	$ 117,217
Fully diluted earnings per share (in usd per share)		$ 0.58	$ 0.64	$ 0.85
Fully diluted weighted-average shares of common stock outstanding (in shares)		185,016,000	163,810,000	138,398,000
NEWMARK Group Inc Parent
Revenues:
Interest income		$ 39,069	$ 27,249	$ 1,433
Revenues		39,069	27,249	1,433
Expenses:
Professional and consulting fees		763	277	0
Interest expense		39,069	27,249	1,499
Other expenses		930	344	0
Total operating expenses		40,762	27,870	1,499
Income before income taxes and noncontrolling interests		(1,693)	(621)	(66)
Equity income of subsidiaries		168,358	190,826	199,166
Provision for income taxes		49,360	83,473	54,608
Net income available to common stockholders		117,305	106,732	144,492
Basic earnings per share
Net income available to common stockholders		$ 104,406	$ 101,641	$ 144,492
Basic earnings per share (in usd per share)		$ 0.59	$ 0.65	$ 1.08
Basic weighted-average shares of common stock outstanding (in shares)		177,774,000	157,256,000	133,413,000
Fully diluted earnings per share
Net income for fully diluted shares		$ 108,160	$ 105,571	$ 117,217
Fully diluted earnings per share (in usd per share)		$ 0.58	$ 0.64	$ 0.85
Fully diluted weighted-average shares of common stock outstanding (in shares)		185,016,000	163,810,000	138,398,000

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