Financial Guarantee Liability - Additional Information (Detail) - USD ($)	9 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Credit Risk [Member]
Guarantee Obligations [Line Items]
Maximum pre-credit enhancement loss exposure	$ 5,700,000,000	$ 5,800,000,000
Credit Risk [Member] \| Fannie Mae and Freddie Mac [Member]
Guarantee Obligations [Line Items]
Outstanding principal balances of credit risk loans being serviced	19,900,000,000	20,600,000,000
Maximum pre-credit enhancement loss exposure	5,700,000,000	5,800,000,000
Credit Risk [Member] \| Fannie Mae and Freddie Mac [Member] \| Credit Enhancement Agreement [Member]
Guarantee Obligations [Line Items]
Maximum pre-credit enhancement loss exposure	$ 9,800,000	$ 76,200,000
Fannie Mae DUS or Freddie TAH Loans [Member] \| Maximum [Member]
Guarantee Obligations [Line Items]
Percentage of contingent liability of actual losses incurred on outstanding loans	33.00%

Percentage of contingent liability of actual losses incurred on outstanding loans.

No definition available.

Name:	nmrk_PercentageOfContingentLiabilityOfActualLossesIncurredOnOutstandingLoans
Namespace Prefix:	nmrk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

For entities that provide full or partial financing and servicing for customers supplementary to its major operations (such as a manufacturer or a homebuilder providing and servicing loans to buyers), represents the liabilities associated with such activities, including accounts payable and accrued liabilities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.15)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: 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)
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_GuaranteeObligationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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

- Details

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

- Details

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

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=nmrk_CreditEnhancementAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nmrk_FannieMaeDUSOrFreddieTargetedAffordableHousingLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: