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Statutory Information - Surplus contributions and dividends of insurance subsidiaries (Details) - Mortgage Guaranty Insurance Corporation - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Related Party Transaction [Line Items] |  |  |
Dividends paid to the parent company | $ 800,000 | $ 400,000 | $ 390,000
Dividend to parent, distribution of investment in subsidiary |  | $ 8,900 |
Dividend to parent, distribution of interest in 9% Debentures |  |  | $ 139,500

X
- Definition Amount of distribution, in form of a dividend, to holding company of interest in convertible junior subordinated debentures. + References No definition available. + Details
Name: | mtg_DividendDistributionOfInterestOfConvertibleJuniorSubordinatedDebentures
Namespace Prefix: | mtg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of distribution, in form of a dividend, to holding company of investment in subsidiary. + References No definition available. + Details
Name: | mtg_DividendDistributionOfInvestmentInSubsidiary
Namespace Prefix: | mtg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition This element represents disclosure of the total aggregate cash dividends paid to the entity by consolidated subsidiaries, by unconsolidated subsidiaries, and by 50 percent or less owned persons accounted for using the equity method. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04(b)) -URI https://asc.fasb.org/extlink&oid=120395691&loc=d3e24072-122690 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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- Details
Name: | dei_LegalEntityAxis=mtg_MortgageGuarantyInsuranceCorporationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |