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Statutory Information - Surplus contributions and dividends of insurance subsidiaries (Details) - Mortgage Guaranty Insurance Corporation - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Related Party Transaction [Line Items] |  |  |  |
Dividends paid to the parent company |  | $ 390,000 | $ 280,000 | $ 220,000
Dividend to parent, distribution of interest in 9% Debentures | $ 139,500 |  |  |

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- Definition Amount of distribution, in form of a dividend, to holding company of interest in convertible junior subordinated debentures. + References No definition available. + Details
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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% 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 http://asc.fasb.org/extlink&oid=26873400&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: |