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Nature of Business (Details) $ in Billions | Dec. 31, 2017 USD ($)
--- | ---
Nature of Business [Abstract] |
Direct domestic primary insurance in force | $ 194.9
Direct domestic primary risk in force | $ 50.3

X
- Definition The principal balance in a Company's records of all mortgage loans it insures. + References No definition available. + Details
Name: | mtg_DirectDomesticPrimaryInsuranceInForce
Namespace Prefix: | mtg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The insurance in force multiplied by the insurance coverage percentage. + References No definition available. + Details
Name: | mtg_DirectDomesticPrimaryRiskInForce
Namespace Prefix: | mtg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | mtg_NatureOfBusinessAbstract
Namespace Prefix: | mtg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration