Compensation - Grant of Conversion Rights to Newmark Employees (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
BGC Holdings, L.P.
Share-based Compensation Arrangement by Share-based Payment Award
Estimated fair value of limited partnership units	$ 58,568	$ 51,378
NEWMARK Group Inc Parent
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units outstanding (in shares)	64,932,884	60,800,852	44,733,487
Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Notional value	$ 193,278	$ 194,995
Estimated fair value of limited partnership units	$ 187,646	$ 182,800

Notional Value Of Limited Partnership Units Eligible For Conversion

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- Definition

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

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

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