Compensation - Limited Partnership Units with a Post-Termination Payout (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award
Liability balance acquisition related post-termination units	$ 6,800
BGC Holdings, L.P.
Share-based Compensation Arrangement by Share-based Payment Award
Notional value	253,730	$ 261,025
Estimated fair value of the post-termination payout	$ 58,568	$ 51,378
Outstanding limited partnership units (in shares)	5,397,109	6,251,816
Outstanding limited partnership units - unvested (in dollars per share)	1,133,784	1,508,510
NEWMARK Group Inc Parent
Share-based Compensation Arrangement by Share-based Payment Award
Outstanding limited partnership units (in shares)	17,820,912	17,097,639
Outstanding limited partnership units - unvested (in dollars per share)	9,028,859	9,357,822

Liability Balance Acquisition Related Post Termination Units

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- Definition

Notional value of limited partnership units with post termination pay out amount.

+ References

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- Definition

Share based compensation arrangement by share based payment award equity instruments other than options outstanding grant date fair value.

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- Definition

The number of limited partner units outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 5
-Subparagraph (SAB Topic 4.F)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187171-122770

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 272
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70434-108055

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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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

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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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