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Derivative Financial Instruments - Narrative (Details) - USD ($) $ in Millions | Jun. 30, 2021 | Dec. 31, 2020
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Other assets |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Collateral deposit assets (liabilities) | $ 2 | $ 61

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

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

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- Details
Name: | us-gaap_BalanceSheetLocationAxis=us-gaap_OtherAssetsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |