Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Consolidated net income (loss)	$ 162,304	$ 110,548	$ 309,725	$ 239,395	$ 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			65,683	61,201
Amortization of mortgage servicing rights, net			55,141	53,001
Employee loan amortization and reserves on employee loans			23,714	43,227
Equity-based compensation and allocations of net income to limited partnership units and FPUs			238,409	185,135
Deferred compensation expense			5,553	7,265
Gain on originated mortgage servicing rights			(65,632)	(98,814)
Unrealized losses (gains) on loans held for sale			(461)	(507)
Loan originations - loans held for sale			(5,937,964)	(7,314,794)
Loan sales - loans held for sale			5,168,395	7,726,805
Losses (gains) on equity method investments	(1,344)	(2,147)	(9,999)	(3,986)
Amortization of discount (premium) on notes payable			3,437	(1,912)
Unrealized loss (gain) on marketable securities			35	(23,038)
Unrealized loss (gain) on derivative asset			(6,327)
Impairment of fixed assets, intangible assets and investments			2,568	8,499
Deferred tax provision (benefit)			9,748	2,263
Recognition of earn-out			(85,135)	(76,969)
Realized losses (gains) on marketable securities			(12,448)	(1,222)
Change in estimated acquisition earn-out payables			(2,009)	5,946
Forfeitures of Class A common stock			(1,219)	(672)
Loss (gain) on other investments			(38,972)
Other				(556)
Consolidated net income (loss), adjusted for non-cash and non-operating items			(277,758)	810,267
Decrease (increase) in operating assets:
Reverse repurchase agreements				54,659
Securities owned			(42,904)	3,236
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers			(1,962,994)	(1,789,576)
Mortgage servicing rights, net				(506)
Accrued commissions and other receivables, net			(154,269)	(99,138)
Loans, forgivable loans and other receivables from employees and partners, net			(156,185)	(82,549)
Receivables from related parties			(2,636)	(3,566)
Other assets			(37,676)	(17,586)
Increase (decrease) in operating liabilities:
Repurchase agreements			198	1,803
Securities loaned			(136,025)	135,070
Accrued compensation			26,262	(37,275)
Payables to broker-dealers, clearing organizations, customers and related broker-dealers			1,897,839	1,731,395
Payables to related parties			26,828	(6,493)
Accounts payable, accrued and other liabilities			61,266	89,176
Net cash provided by (used in) operating activities			(758,054)	788,917
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of fixed assets			(25,656)	(28,998)
Capitalization of software development costs			(38,487)	(27,841)
Purchase of equity and cost method investments			(23,099)	(101,623)
Proceeds from equity method investments			10,064	243
Payments for acquisitions, net of cash and restricted cash acquired			(33,573)	(22,270)
Advances to related parties				(375,000)
Repayments from related parties				375,000
Purchase of mortgage servicing rights			(2,124)
Proceeds from sale of marketable securities			153,251	7,849
Capitalization of trademarks, patent defense and registration costs				(295)
Net cash (used in) provided by investing activities			40,376	(172,935)
CASH FLOWS FROM FINANCING ACTIVITIES:
Repayments of notes payable and other borrowings			(970,160)	(7,060)
Issuance of long-term debt and collateralized borrowings, net of deferred issuance costs			639,241	998,817
Proceeds from warehouse notes payable			5,937,964	7,314,793
Repayments on warehouse notes payable			(5,166,612)	(6,913,030)
Proceeds from issuance of exchangeable preferred partnership units in Newmark Partners, L.P.			266,093
Advances from related parties				291,000
Repayments to related parties				(981,000)
Earnings distributions			(182,731)	(89,537)
Redemption and repurchase of limited partnership interests			(29,421)	(34,113)
Dividends to stockholders			(176,228)	(148,094)
Repurchase of Class A common stock			(9,806)	(14,557)
Cancellation of restricted stock units in satisfaction of withholding tax requirements			(647)	(52)
Proceeds from issuance of Class A common stock, net of costs			327,624
Loan from related parties			80,000
Acquisition of Berkeley Point				(871,814)
Pre-acquisition distribution from Berkeley Point to CCRE				(66,782)
Proceeds from short-term borrowings				6,313
Payments on acquisitions earn-outs			(11,180)	(12,211)
Net cash (used in) provided by financing activities			704,137	(527,327)
Effect of exchange rate changes on cash and cash equivalents, Restricted cash and Cash segregated under regulatory requirements			(7,831)	3,853
Net (decrease) increase in cash and cash equivalents, Restricted cash and Cash segregated under regulatory requirements			(21,372)	92,508
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	$ 775,418	$ 685,943	775,418	685,943	$ 796,790
Supplemental cash information:
Cash paid during the period for taxes			38,438	32,136
Cash paid during the period for interest			71,413	68,331
Supplemental non-cash information:
Issuance of Class A common stock upon exchange of limited partnership interests			127,322	74,013
Issuance of Class A and contingent Class A common stock and limited partnership interests for acquisitions			19,732	14,232
Distribution from Berkeley Point to CCRE				2,993
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Supplemental non-cash information:
Shares received			$ 85,135	$ 76,969