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Credit Enhancement Receivable, Credit Enhancement Deposit and Contingent Liability - Narrative (Details) - USD ($)		3 Months Ended		6 Months Ended
	Mar. 23, 2021	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 09, 2021	Dec. 31, 2020
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit
Credit enhancement receivable		$ 0		$ 0			$ 0
Deposit in Fannie Mae restricted liquidity account		25,000,000.0		25,000,000.0		$ 25,000,000.0	25,000,000.0
Returned the credit enhancement deposit	$ 25,000,000.0
DB Cayman | Credit Risk
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit
Reimbursements under serving agreement		0	$ 0	0	$ 0
Percentage of contingent payment (in percentage)						50.00%
Contingent liability		$ 0		$ 0			$ 12,300,000

X

- Definition

Credit enhancement receivable.

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	instant

X

- Definition

Credit enhancement receivable contingent liability and credit enhancement deposit.

+ References

No definition available.

+ Details

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- Definition

Percentage of contingent payment.

+ References

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- Definition

Reimbursements under serving agreement.

+ References

No definition available.

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Balance Type:	credit
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- Definition

Repayment Of Security Deposit

+ References

No definition available.

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Namespace Prefix:	nmrk_
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Balance Type:	credit
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- Definition

Amount of loss contingency liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349

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- Definition

The amount of an asset, typically cash, provided to a counterparty to provide certain assurance of performance by the entity pursuant to the terms of a written or oral agreement, such as a lease.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8,17)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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- Details

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