Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Consolidated net income	$ 32,487	$ 37,007	$ 145,096
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on originated mortgage servicing rights	(6,389)	(28,806)
Depreciation and amortization	22,513	18,237
Equity-based compensation and allocations of net income to limited partnership units	25,809
Employee loan amortization and impairment	6,009	1,974
Change in fair value of contingent consideration	134
Unrealized gains on loans held for sale	(15,126)	(2,102)
Income from an equity method investment	(3,176)
Amortization of deferred financing costs	255	259
Provision for uncollectible accounts	1,140	(211)
Realized gain on marketable securities	(2,400)
Unrealized gain on marketable securities	(796)
Loan originations—loans held for sale	(1,410,690)	(1,842,357)
Loan sales—loans held for sale	822,811	2,139,634
Consolidated net income, adjusted for non-cash and non-operating items	(527,419)	323,635
Changes in operating assets and liabilities:
Receivables, net	(19,374)	4,340
Loans, forgivable loans and other receivables from employees and partners	(24,583)	(6,226)
Other assets	(14,063)	10,498
Accrued compensation	(35,069)	(33,599)
Accounts payable, accrued expenses and other liabilities	30,114	(2,237)
Net cash provided by (used in) operating activities	(590,394)	296,411
CASH FLOWS FROM INVESTING ACTIVITIES:
Cash acquired, net of purchases of noncontrolling interest		(997)
Proceeds from the sale of marketable securities	52,196
Investment in cost method investments	(7,500)
Purchases of fixed assets	(1,714)	(3,053)
Purchase of mortgage servicing rights	(509)
Net cash provided (used in) by investing activities	42,473	(4,050)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from warehouse notes payable	1,410,690	1,842,357
Principal payments on warehouse notes payable	(820,651)	(1,879,963)
Proceeds from BGC's purchase of exchangeability limited partnership units in Newmark Holdings	241,960
Payments to related parties	(13,000)	(578,263)
Borrowings from related parties	177,016	354,493
Repayment of long-term debt	(270,710)
Payments for IPO offering costs	(8,870)
Secured loans	(49,001)
Distributions to noncontrolling interests	(100)	(71)
Payments on acquisition earn-outs	(758)	(10,513)
Payment of deferred financing costs	(16)	(9)
Net cash provided (used in) by financing activities	666,560	(271,969)
Net increase (decrease) in cash and cash equivalents	118,639	20,392
Cash and cash equivalents and restricted cash at beginning of period	173,374	117,554	117,554
Cash and cash equivalents and restricted cash at end of period	292,013	137,946	173,374
Cash paid during the period for:
Interest	12,922	2,969
Taxes	19	$ 18
Cash and cash equivalents	48,069		121,027
Restricted cash	$ 243,944		$ 52,347