Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Consolidated net income (loss)	$ 49,990	$ 77,731	$ 147,421	$ 128,847	$ 81,729
Adjustments to reconcile consolidated net income (loss) to net cash provided by (used in) operating activities:
Fixed asset depreciation and intangible asset amortization			43,047	41,078
Amortization of mortgage servicing rights, net			33,925	33,161
Employee loan amortization and reserves on employee loans			16,769	17,194
Equity-based compensation and allocations of net income to limited partnership units and FPUs			167,342	128,560
Deferred compensation expense			4,223	6,208
Gain on originated mortgage servicing rights			(31,731)	(69,265)
Unrealized losses (gains) on loans held for sale			(7,288)	(2,534)
Loan originations - loans held for sale			(2,659,827)	(5,811,773)
Loan sales - loans held for sale			2,481,782	5,952,293
Losses (gains) on equity method investments	(2,854)	(1,602)	(8,655)	(1,839)
Amortization of discount (premium) on notes payable			(470)	(458)
Unrealized (gain) loss on marketable securities			(5,726)	(10,243)
Unrealized loss on derivative asset			2,808
Impairment of fixed assets, intangible assets and investments			1,900	1,638
Deferred tax provision (benefit)			7,121	2,975
Realized losses (gains) on marketable securities			(10,787)	(1,222)
Change in estimated acquisition earn-out payables			(980)	(3,770)
Loss (gain) on other investments			(21,207)	(224)
Other			(1,761)	(932)
Consolidated net income (loss), adjusted for non-cash and non-operating items			157,906	409,694
Decrease (increase) in operating assets:
Reverse repurchase agreements				54,659
Securities owned			(24,407)	1,614
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers			(969,177)	(1,147,417)
Accrued commissions and other receivables, net			(117,435)	(78,145)
Loans, forgivable loans and other receivables from employees and partners, net			(91,732)	(47,704)
Receivables from related parties			(2,169)	(4,099)
Other assets			(27,832)	(18,486)
Increase (decrease) in operating liabilities:
Repurchase agreements			3,108
Securities loaned			(124,839)	95,327
Accrued compensation			(5,059)	(28,714)
Payables to broker-dealers, clearing organizations, customers and related broker-dealers			909,570	1,108,124
Payables to related parties			36,632	9,553
Accounts payable, accrued and other liabilities			61,278	82,473
Net cash provided by (used in) operating activities			(194,156)	436,879
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of fixed assets			(21,347)	(21,571)
Capitalization of software development costs			(19,927)	(14,341)
Purchase of equity and cost method investments			(22,867)	(888)
Proceeds from equity method investments			5,724	243
Payments for acquisitions, net of cash and restricted cash acquired			(6,190)	14,434
Advances to related parties				(285,000)
Repayments from related parties				155,000
Purchase of mortgage servicing rights			(1,608)
Proceeds from sale of marketable securities			146,581	7,829
Capitalization of trademarks, patent defense and registration costs				(248)
Net cash (used in) provided by investing activities			80,366	(144,542)
CASH FLOWS FROM FINANCING ACTIVITIES:
Repayments of notes payable and other borrowings			(430,794)	(3,523)
Proceeds from notes payable and other borrowings			70,000	29,907
Proceeds from warehouse notes payable			2,659,827	5,851,890
Repayments on warehouse notes payable			(2,479,696)	(5,175,950)
Proceeds from issuance of exchangeable preferred partnership units in Newmark Partners, L.P.			152,886
Advances from related parties				241,000
Repayments to related parties				(931,000)
Earnings distributions			(129,235)	(68,304)
Redemption and repurchase of limited partnership interests			(19,383)	(24,156)
Dividends to stockholders			(113,133)	(96,510)
Repurchase of Class A common stock			(9,711)	(10,590)
Cancellation of restricted stock units in satisfaction of withholding tax requirements			(647)	(52)
Proceeds from issuance of Class A common stock, net of costs			324,177
Loan from related parties			130,000
Proceeds from short-term borrowings				150,000
Loans to related parties				(150,000)
Payments on acquisitions earn-outs			(7,252)	(10,509)
Net cash (used in) provided by financing activities			147,039	(197,797)
Effect of exchange rate changes on cash and cash equivalents, Restricted cash and Cash segregated under regulatory requirements			(8,014)	7,686
Net (decrease) increase in cash and cash equivalents, Restricted cash and Cash segregated under regulatory requirements			25,235	102,226
Cash and cash equivalents, Restricted cash and Cash segregated under regulatory requirements at beginning of period			796,790	593,435	593,435
Cash and cash equivalents, Restricted cash and Cash segregated under regulatory requirements at end of period	$ 822,025	$ 695,661	822,025	695,661	$ 796,790
Supplemental cash information:
Cash paid during the period for taxes			24,208	20,782
Cash paid during the period for interest			51,537	45,467
Supplemental non-cash information:
Issuance of Class A common stock upon exchange of limited partnership interests			99,901	56,443
Issuance of Class A and contingent Class A common stock and limited partnership interests for acquisitions			$ 7,134	$ 4,225