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Derivative Financial Instruments - Narrative (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Dec. 31, 2019
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Derivative Instruments and Hedging Activities Disclosure [Abstract] |  |
Collateral deposit assets (liabilities) | $ 14 | $ 6

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- Definition Margin Deposit Assets (Liabilities) + References No definition available. + Details
Name: | nsm_MarginDepositAssetsLiabilities
Namespace Prefix: | nsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration