Line of credit facility capacity available based on temporary amount.

No definition available.

Name:	nmrk_LineOfCreditFacilityCapacityAvailableBasedOnTemporaryAmount
Namespace Prefix:	nmrk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of temporary increase (decrease) of the credit facility.

+ References

No definition available.

+ Details

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

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The maximum amount of borrowing capacity under a line of credit that is available as of the balance sheet date for a specific purpose other than for financing goods acquired for inventory or imminent delivery to a customer.

+ References

No definition available.

+ Details

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

- Definition

Amount of increase (decrease) of the credit facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_LineOfCreditFacilityIncreaseDecreaseForPeriodNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Warehouse Facilities Collateralized by U.S. Government Sponsored Enterprises - Schedule of Company Lines Available and Borrowings Outstanding (Parenthetical) (Detail) - USD ($) $ in Millions	1 Months Ended	2 Months Ended				6 Months Ended	12 Months Ended
	Aug. 28, 2019	Sep. 13, 2019	Jul. 15, 2019	Mar. 31, 2019	Jan. 29, 2019	Jun. 30, 2019	Dec. 31, 2018
Warehouse Facility Due June 17, 2020 [Member]
Securities Financing Transaction [Line Items]
Maturity date						Jun. 17, 2020
Warehouse Facility Due June 17, 2020 [Member]
Securities Financing Transaction [Line Items]
Maturity date						Jun. 17, 2020
Warehouse Facility Due September 25, 2019 [Member]
Securities Financing Transaction [Line Items]
Maturity date						Sep. 25, 2019	Sep. 25, 2019
Warehouse Facility Due October 10, 2019 [Member]
Securities Financing Transaction [Line Items]
Maturity date						Oct. 10, 2019	Oct. 10, 2019
Warehouse facility available					$ 1,000.0
Warehouse facility increase (decrease)					$ 700.0
Facility available based on temporary amount				$ 300.0
Temporary facility increase (decrease) amount				$ (400.0)
Warehouse Facility Due October 10, 2019 [Member] \| Scenario, Forecast [Member]
Securities Financing Transaction [Line Items]
Warehouse facility available	$ 500.0	$ 1,800.0
Warehouse facility increase (decrease)	$ 200.0	$ 1,300.0
Warehouse Facility Due October 10, 2019 [Member] \| Subsequent Event [Member]
Securities Financing Transaction [Line Items]
Warehouse facility available			$ 700.0
Warehouse facility increase (decrease)			$ 400.0
Warehouse Facility Due June 19, 2019 [Member]
Securities Financing Transaction [Line Items]
Maturity date							Jun. 19, 2019