Compensation - Activity Associated with Limited Partnership Units (Detail) - shares	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2019
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)	60,800,852	44,733,487
Number of Units, Granted (in shares)	6,811,808	13,813,204
Number of units, Redeemed/exchanged units (in shares)	(1,798,651)	(2,487,885)
Number of Units, Forfeited units (in shares)	(82,945)	4,742,046
Number of Units, Balance outstanding at end of period (in shares)	65,731,064	60,800,852
Exchangeable Units (units)	10,017,066	10,108,598
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)	56,053,701	61,870,969
Number of Units, Granted (in shares)	1,064,781	319,586
Number of units, Redeemed/exchanged units (in shares)	(2,217,536)	(3,938,134)
Number of Units, Forfeited units (in shares)	(40,451)	(2,198,720)
Number of Units, Balance outstanding at end of period (in shares)	54,860,495	56,053,701
Exchangeable Units (units)	25,047,827	24,692,695
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)	61,301,972
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)	53,247,827
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)	4,429,092
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)	1,612,668

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

+ Details

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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

+ Details

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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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