Compensation - Activity Associated with Restricted Stock Units (Detail) - Restricted Stock Units (RSUs) [Member] - $ / shares	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Newmark Holdings, L.P. [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding at beginning of period	219,887
Number of Units, Granted	3,110,510	264,532
Number of Settled units (delivered shares)	(65,420)	(8,109)
Number of Units, Forfeited units	(86,802)	(36,536)
Number of Units, Balance outstanding at end of period	3,178,175	219,887
Balance at the beginning of period	$ 13.52
Granted	7.39	$ 13.54
Settled units (delivered shares)	13.53	13.36
Forfeited units	9.37	13.71
Balance at the end of period	$ 7.63	$ 13.52
Weighted-Average Remaining Contractual Term (Years)	5 years 1 month 13 days	2 years 3 months 10 days
BGC Holdings, L.P. [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding at beginning of period	168,675	346,538
Number of Units, Granted		3,439
Number of Settled units (delivered shares)	(96,520)	(147,006)
Number of Units, Forfeited units	(11,290)	(34,296)
Number of Units, Balance outstanding at end of period	60,865	168,675	346,538
Balance at the beginning of period	$ 9.77	$ 9.56
Granted		7.64
Settled units (delivered shares)	9.38	9.17
Forfeited units	10.02	10.01
Balance at the end of period	$ 10.35	$ 9.77	$ 9.56
Weighted-Average Remaining Contractual Term (Years)	5 months 12 days	11 months 23 days	1 year 10 months 6 days

Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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- Definition

Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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- Definition

Number of non-option equity instruments exercised by participants.

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Number of shares under non-option equity instrument agreements that were cancelled as a result of occurrence of a terminating event.

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Net number of non-option equity instruments granted to participants.

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- Definition

Number of equity instruments other than options outstanding, including both vested and non-vested instruments.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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