Schedule I - Parent Company Only Financial Statements - Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Consolidated net income	$ 117,305	$ 106,732	$ 144,492
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Equity income from subsidiaries	(7,250)	(2,750)	(1,562)
Deferred tax provision/(benefit)	(27,852)	16,387	44,383
Changes in operating assets and liabilities:
Other assets	(113,241)	(9,924)	36,086
Accounts payable, accrued expenses and other liabilities	90,712	152,885	13,103
Net cash provided by (used in) operating activities	986,760	(332,367)	853,637
CASH FLOWS FROM INVESTING ACTIVITIES:
Net cash (used in) provided by investing activities	(56,788)	7,742	379
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from the Initial Public Offering, net of underwriting discounts	0	(8,870)	304,290
Dividends to stockholders	0	0	(101,731)
Treasury stock repurchases	(37,368)	(486)	0
Net cash (used in) provided by financing activities	(895,506)	338,657	(798,196)
Net increase in cash and cash equivalents and restricted cash	34,466	14,032	55,820
Cash and cash equivalents and restricted cash at beginning of period	187,406	173,374	117,554
Cash and cash equivalents and restricted cash at end of period	221,872	187,406	173,374
Cash paid during the period for:
Interest	36,959	81,838	21,003
Taxes	95,089	1,165	46
Supplemental disclosure of noncash investing and financing activities:
Treasury stock repurchases	4,732	0	0
Accrued offering costs	0	0	8,870
NEWMARK Group Inc Parent
CASH FLOWS FROM OPERATING ACTIVITIES:
Consolidated net income	117,305	106,732	144,492
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Equity income from subsidiaries	(168,358)	(190,826)	(199,166)
Deferred tax provision/(benefit)	(29,127)	14,197	47,548
Changes in operating assets and liabilities:
Receivables from subsidiaries	4,223	22,717	(2,342)
Payable to subsidiaries	(7,539)	120,483	1,157
Other assets	2,711	(1,655)	0
Accounts payable, accrued expenses and other liabilities	(9,873)	68,123	8,311
Net cash provided by (used in) operating activities	(90,658)	139,771	0
CASH FLOWS FROM INVESTING ACTIVITIES:
Cash paid for acquisitions	(17,726)	(6,691)	0
Contribution to subsidiary	0	0	(304,290)
Net cash (used in) provided by investing activities	(17,726)	(6,691)	(304,290)
CASH FLOWS FROM FINANCING ACTIVITIES:
Distributions from subsidiaries	236,074	107,000	0
Proceeds from the Initial Public Offering, net of underwriting discounts	0	0	304,290
Repayment of debt	(105,000)	(670,710)	(304,290)
Borrowing of debt	155,000	537,926	0
Reinvestment of cash in subsidiaries	(71,000)	(65,000)	0
Dividends to stockholders	(69,245)	(41,787)	0
Treasury stock repurchases	(37,368)	(486)	0
Repayment of related party receivable	0	0	304,290
Net cash (used in) provided by financing activities	108,461	(133,057)	304,290
Net increase in cash and cash equivalents and restricted cash	77	23	0
Cash and cash equivalents and restricted cash at beginning of period	24	1	1
Cash and cash equivalents and restricted cash at end of period	101	24	1
Cash paid during the period for:
Interest	36,959	21,751	0
Taxes	90,813	1,165	0
Supplemental disclosure of noncash investing and financing activities:
Treasury stock repurchases	4,732	0	0
Net assets contributed by BGC Partners’	0	0	795,497
Note receivable from related parties	0	0	975,000
Debt assumed from BGC	0	0	975,000
Accrued offering costs	$ 0	$ 0	$ 8,870