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v3.21.1

Indebtedness - Schedule of Notes Maturity (Details) - Unsecured Senior Notes - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Debt Instrument [Line Items] |  |
2021 through 2025 | $ 0 |
Thereafter | 2,100 |
Total unsecured senior notes principal amount | $ 2,100 | $ 2,100

X
- Definition Long-Term Debt, Maturity, after Year Four + References No definition available. + Details
Name: | nsm_LongTermDebtMaturityAfterYearFour
Namespace Prefix: | nsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Long-Term Debt, Maturity, Years One Through Four + References No definition available. + Details
Name: | nsm_LongTermDebtMaturityYearsOneThroughFour
Namespace Prefix: | nsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Unsecured Debt, Gross + References No definition available. + Details
Name: | nsm_UnsecuredDebtGross
Namespace Prefix: | nsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- 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

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- Details
Name: | us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |