Schedule I - Parent Company Only Financial Statements - Statements of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Consolidated net income	$ 161,712	$ 109,277	$ 161,712	$ 191,898
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Loss (income) from an equity method investment		11,562	(7,250)	(2,750)
Deferred tax (benefit) provision		419	(27,852)	16,387
Changes in operating assets and liabilities:
Payable to subsidiaries		(25,989)	23,433	0
Other assets		21,764	(93,823)	(9,924)
Accounts payable, accrued expenses and other liabilities		(82,415)	17,829	152,885
Net cash (used in) provided by operating activities		(777,694)	986,761	(332,367)
CASH FLOWS FROM INVESTING ACTIVITIES:
Payments for acquisitions, net of cash acquired		(5,850)	(33,939)	(34,513)
Net cash (used in) provided by investing activities		3,602	56,788	(7,742)
CASH FLOWS FROM FINANCING ACTIVITIES:
Repayment of long-term debt		(275,000)	(105,000)	(670,710)
Dividends to stockholders		(23,171)	(69,245)	(41,787)
Treasury stock repurchases		(6,364)	(37,368)	(486)
Net cash provided by (used in) financing activities		817,823	(895,506)	338,657
Net increase in cash and cash equivalents and restricted cash		36,527	34,466	14,032
Cash and cash equivalents and restricted cash at beginning of period		221,872	187,406	173,374
Cash and cash equivalents and restricted cash at end of period	221,872	258,399	221,872	187,406
Cash paid during the period for:
Interest		40,640	36,959	81,838
Taxes		80,288	95,089	1,165
Supplemental disclosure of non-cash operating, investing and financing activities:
Treasury stock repurchase		453	4,732	0
NEWMARK Group Inc Parent
CASH FLOWS FROM OPERATING ACTIVITIES:
Consolidated net income		80,060	117,305	106,732
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Loss (income) from an equity method investment		(113,732)	(168,358)	(190,826)
Deferred tax (benefit) provision		1,337	(29,127)	14,197
Changes in operating assets and liabilities:
Receivables from subsidiaries		(105,356)	4,223	22,717
Payable to subsidiaries		(30,906)	(7,539)	120,483
Other assets		(6,367)	2,711	(1,655)
Accounts payable, accrued expenses and other liabilities		(43,845)	(9,873)	68,123
Net cash (used in) provided by operating activities		(218,809)	(90,658)	139,771
CASH FLOWS FROM INVESTING ACTIVITIES:
Payments for acquisitions, net of cash acquired		(5,850)	(17,726)	(6,691)
Net cash (used in) provided by investing activities		(5,850)	(17,726)	(6,691)
CASH FLOWS FROM FINANCING ACTIVITIES:
Distributions from subsidiaries		531,938	236,074	107,000
Repayment of long-term debt		(275,000)	(105,000)	(670,710)
Borrowings of long-term debt		365,000	155,000	537,926
Reinvestment of cash in subsidiaries		(365,000)	(71,000)	(65,000)
Dividends to stockholders		(23,171)	(69,245)	(41,787)
Treasury stock repurchases		(6,817)	(37,368)	(486)
Payment of deferred financing costs		(2,316)	0	0
Net cash provided by (used in) financing activities		224,634	108,461	(133,057)
Net increase in cash and cash equivalents and restricted cash		(25)	77	23
Cash and cash equivalents and restricted cash at beginning of period		101	24	1
Cash and cash equivalents and restricted cash at end of period	$ 101	76	101	24
Cash paid during the period for:
Interest		40,640	36,959	21,751
Taxes		76,446	90,813	1,165
Supplemental disclosure of non-cash operating, investing and financing activities:
Treasury stock repurchase		$ 453	$ 4,732	$ 0