Compensation - Activity Associated with Limited Partnership Units (Detail) - shares	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
BGC Holdings, L.P.
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Number of Units, Balance outstanding at beginning of period (in shares)	61,870,969	64,708,915	53,407,627
Number of Units, Granted (in shares)	319,586	2,872,825	13,976,871
Number of units, Redeemed/exchanged units (in shares)	(3,938,134)	(5,650,292)	(2,668,048)
Number of Units, Forfeited units (in shares)	(2,198,720)	(60,479)	(7,535)
Number of Units, Balance outstanding at end of period (in shares)	56,053,701	61,870,969	64,708,915
Exchangeable Units	24,692,695	25,999,302
NEWMARK Group Inc Parent
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Number of Units, Balance outstanding at beginning of period (in shares)	44,733,487	29,413,143
Number of Units, Granted (in shares)	13,813,204	19,141,943
Number of units, Redeemed/exchanged units (in shares)	(2,487,885)	(3,793,351)
Number of Units, Forfeited units (in shares)	(4,742,046)	28,248
Number of Units, Balance outstanding at end of period (in shares)	60,800,852	44,733,487	29,413,143
Exchangeable Units	10,108,598	9,251,052
Regular units \| BGC Holdings, L.P.
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Number of Units, Balance outstanding at end of period (in shares)	54,500,896
Regular units \| NEWMARK Group Inc Parent
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Number of Units, Balance outstanding at end of period (in shares)	56,932,154
Preferred units \| BGC Holdings, L.P.
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Number of Units, Balance outstanding at end of period (in shares)	1,552,805
Preferred units \| NEWMARK Group Inc Parent
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Number of Units, Balance outstanding at end of period (in shares)	3,868,698

Exchangeable Units

No definition available.

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- Definition

Number of exchangeable limited partnership units redeemed or exchanged into the Company's class A common stock during the period.

+ References

No definition available.

+ Details

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- Definition

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

+ References

+ Details

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- Definition

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

+ References

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- Definition

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

+ References

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

Reference 2: 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)(i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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