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Reinsurance - Excess of Loss Reinsurance (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
Home Re 2018-1 |  |
Effects of Reinsurance [Line Items] |  |
Remaining First Layer Retention | $ 167,779 | $ 168,691
Remaining Excess of Loss Reinsurance Coverages | 260,957 | 318,636
Home Re 2019-1 |  |
Effects of Reinsurance [Line Items] |  |
Remaining First Layer Retention | 185,636 | 0
Remaining Excess of Loss Reinsurance Coverages | 271,021 | 0
Home Re special purpose insurers |  |
Effects of Reinsurance [Line Items] |  |
Remaining First Layer Retention | 353,415 | 168,691
Remaining Excess of Loss Reinsurance Coverages | $ 531,978 | $ 318,636

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- Definition Reinsurance Retention Policy, Remaining Excess of Loss Reinsurance Coverages + References No definition available. + Details
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Namespace Prefix: | mtg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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_
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- Definition Stated amount that is the upper retention limit per entity policy. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
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Namespace Prefix: |
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- Details
Name: | dei_LegalEntityAxis=mtg_HomeRe20191Member
Namespace Prefix: |
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- Details
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Namespace Prefix: |
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Period Type: |