Concentration risk credit risk financial instrument maximum exposure without credit protection.

No definition available.

Name:	nmrk_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposureWithoutCreditProtection
Namespace Prefix:	nmrk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Contingent payment due date.

+ References

No definition available.

+ Details

Name:	nmrk_ContingentPaymentDueDate
Namespace Prefix:	nmrk_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Contingent payments description.

+ References

No definition available.

+ Details

Name:	nmrk_ContingentPaymentsDescription
Namespace Prefix:	nmrk_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Credit enhancement agreement date.

+ References

No definition available.

+ Details

Name:	nmrk_CreditEnhancementAgreementDate
Namespace Prefix:	nmrk_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Credit enhancement receivable.

+ References

No definition available.

+ Details

Name:	nmrk_CreditEnhancementReceivable
Namespace Prefix:	nmrk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Credit enhancement receivable contingent liability and credit enhancement deposit.

+ References

No definition available.

+ Details

Name:	nmrk_CreditEnhancementReceivableContingentLiabilityAndCreditEnhancementDepositLineItems
Namespace Prefix:	nmrk_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of contingent payment.

+ References

No definition available.

+ Details

Name:	nmrk_PercentageOfContingentPayment
Namespace Prefix:	nmrk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Reimbursements under servicing agreement.

+ References

No definition available.

+ Details

Name:	nmrk_ReimbursementsUnderServicingAgreement
Namespace Prefix:	nmrk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Security deposit return date.

+ References

No definition available.

+ Details

Name:	nmrk_SecurityDepositReturnDate
Namespace Prefix:	nmrk_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13537-108611

+ Details

Name:	us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This item represents disclosure of all significant concentrations of credit risk or market risk arising from the subject financial instrument (as defined), whether from an individual counterparty or groups of counterparties.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

+ Details

Name:	us-gaap_FairValueConcentrationOfRiskLoansReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=118943654&loc=d3e12069-110248

+ Details

Name:	us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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=118942415&loc=d3e14326-108349

+ Details

Name:	us-gaap_LossContingencyAccrualAtCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_SecurityDeposit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents money paid in advance to protect the provider of a product or service, such as a lessor, against damage or nonpayment by the buyer or tenant (lessee) during the term of the agreement. Such damages may include physical damage to the property, theft of property, and other contractual breaches. Security deposits held may be interest or noninterest bearing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.15(a))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

+ Details

Name:	us-gaap_SecurityDepositLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherNoncurrentLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ManagingRisksInherentInServicingAssetsAndServicingLiabilitiesByTypeOfRiskAxis=us-gaap_CreditRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=nmrk_DBCaymanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Credit Enhancement Receivable, Contingent Liability and Credit Enhancement Deposit - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit [Line Items]
Credit enhancement agreement date			Mar. 09, 2012
Credit enhancement receivable	$ 0		$ 0		$ 0
Deposit in Fannie Mae restricted liquidity account	25,000,000.0		25,000,000.0
Deposit in Fannie Mae restricted liquidity account	25,000,000		$ 25,000,000		25,000,000
Security deposit return date			Mar. 09, 2021
Other Long-Term Liabilities [Member]
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit [Line Items]
Deposit in Fannie Mae restricted liquidity account	25,000,000.0		$ 25,000,000.0
Credit Risk [Member]
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit [Line Items]
Credit risk loans	19,900,000,000		19,900,000,000		20,600,000,000
Maximum pre-credit enhancement loss exposure	5,700,000,000		5,700,000,000		5,800,000,000
DB Cayman [Member] \| Credit Risk [Member]
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit [Line Items]
Reimbursements under servicing agreement	0	$ 0	0	$ 0
Credit risk loans	$ 29,400,000		29,400,000		230,700,000
Maximum pre-credit enhancement loss exposure			9,800,000		76,200,000
Maximum loss exposure without any form of credit protection			$ 5,700,000,000		5,700,000,000
Percentage of contingent payment	50.00%		50.00%
Contingent payment due date			Mar. 09, 2021
Contingent payments description			Newmark is required to pay DB Cayman, on March 9, 2021, an amount equal to 50% of the positive difference, if any, between (a) $25.0 million, and (b) Newmark’s unreimbursed loss-sharing payments from March 9, 2012 through March 9, 2021 on Newmark’s servicing portfolio as of March 9, 2012.
Contingent liability	$ 11,700,000		$ 11,700,000		$ 11,100,000