Percentage of net cash proceeds of material asset sales.

No definition available.

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

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

+ References

+ Details

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

- Definition

Aggregate amount of interest paid or due on all long-term debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.8)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

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

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

+ Details

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

- Definition

Description of the terms of a credit facility arrangement. Terms typically include interest rate, collateral required, guarantees required, repayment requirements, and restrictions on use of assets and activities of the entity.

+ References

+ Details

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

- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityExpirationDate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The effective interest rate at the end of the reporting period.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

Amount of cash outflow for payment of an obligation from a lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3291-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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_RepaymentsOfLinesOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_LongtermDebtTypeAxis=nmrk_ConvertedTermLoanMember
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_VariableRateAxis=nmrk_LondonInterbankOfferedRateLIBOROrDefinedBaseRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=nmrk_TermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=nmrk_TwoThousandAndNineteenPromissoryNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=nmrk_TwoThousandAndFortyTwoPromissoryNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long-Term Debt and Long-Term Debt Payable to Related Parties - Additional Information (Detail) - USD ($)					3 Months Ended		12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Dec. 13, 2017	Sep. 08, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Converted Term Loan [Member]
Debt Instrument [Line Items]
Maximum revolving credit				$ 400,000,000
Credit agreement maturity date					Sep. 08, 2019
Line of credit facility, description					Borrowings under the Converted Term Loan bear interest at either LIBOR or a defined base rate plus an additional margin, which ranges from 50 basis points to 325 basis points depending on BGC’s debt rating as determined by S&P and Fitch and whether such loan is a LIBOR loan or a base rate loan. Since there were amounts outstanding under the term loan facility as of December 31, 2017, the pricing increased by 50 basis points until the term loan facility is paid in full, and if there are any amounts outstanding under the term loan facility as of June 30, 2018, the pricing shall increase by an additional 75 basis points (125 basis points in the aggregate) until the term loan facility is paid in full. From and after the repayment in full of the term loan facility, the pricing shall return to the levels previously described
Stated Spread to One Month LIBOR		0.50%
Debt instrument gross amount		$ 400,000,000			$ 400,000,000		$ 400,000,000
Current interest rate					3.99%
Interest expense					$ 4,600,000	$ 0
Converted Term Loan [Member] \| Scenario, Forecast [Member]
Debt Instrument [Line Items]
Stated Spread to One Month LIBOR	0.75%
Converted Term Loan [Member] \| LIBOR or Defined Base Rate [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Stated Spread to One Month LIBOR				0.50%
Converted Term Loan [Member] \| LIBOR or Defined Base Rate [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Stated Spread to One Month LIBOR				3.25%
Term Loan [Member]
Debt Instrument [Line Items]
Maximum revolving credit				$ 575,000,000
Credit agreement maturity date					Sep. 08, 2019
Line of credit facility, description					Borrowings under the Term Loan bear interest at either LIBOR or a defined base rate plus an additional margin which ranges from 50 basis points to 325 basis points depending on BGC’s debt rating as determined by S&P and Fitch and whether such loan is a LIBOR loan or a base rate loan. Since there were amounts outstanding under the term loan facility as of December 31, 2017, the pricing increased by 50 basis points until the term loan facility is paid in full and if there are any amounts outstanding under the term loan facility as of June 30, 2018, the pricing shall increase by an additional 75 basis points (125 basis points in the aggregate) until the term loan facility is paid in full. From and after the repayment in full of the term loan facility, the pricing shall return to the levels previously described.
Stated Spread to One Month LIBOR		0.50%
Interest expense					$ 2,600,000
Percentage of net cash proceeds of all material asset sales				100.00%
Repayments of credit agreement			$ 304,300,000		270,700
Line of credit facility, outstanding amount					0
Term Loan [Member] \| Scenario, Forecast [Member]
Debt Instrument [Line Items]
Stated Spread to One Month LIBOR	0.75%
Term Loan [Member] \| LIBOR or Defined Base Rate [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Stated Spread to One Month LIBOR				0.50%
Term Loan [Member] \| LIBOR or Defined Base Rate [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Stated Spread to One Month LIBOR				3.25%
2019 Promissory Notes [Member]
Debt Instrument [Line Items]
Debt instrument gross amount			300,000,000
Interest expense					$ 4,300,000		800,000
Maturity date					Dec. 09, 2019
2042 Promissory Notes [Member]
Debt Instrument [Line Items]
Debt instrument gross amount			$ 112,500,000
Interest expense					$ 2,300,000		$ 500,000
Maturity date					Jun. 26, 2042