Name:	mtg_AnniversaryPaymentReleaseOfStartOfInterestDeferralToAlternativePaymentMechanismInLieuOfFinalMaturityOfDebentures
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. Fair value of convertible debt at the balance sheet date.

No definition available.

Name:	mtg_ConvertibleSeniorNotesDueOneFairValueDisclosure
Namespace Prefix:	mtg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

No definition available.

Name:	mtg_ConvertibleSeniorNotesDueTwoFairValueDisclosure
Namespace Prefix:	mtg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The price per share of the conversion feature embedded in the debt instrument.

No definition available.

Name:	mtg_DebtInstrumentConvertibleConversionPrice2
Namespace Prefix:	mtg_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

The number of shares that can be issued in exchange for the original debt if converted in a noncash (or part noncash) transaction. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or payments in the period.

No definition available.

Name:	mtg_DebtInstrumentConvertibleNumberOfEquityInstrumentsIfDebtConverted
Namespace Prefix:	mtg_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

The amount of cash and investments held at our holding company at balance sheet date.

No definition available.

Name:	mtg_HoldingCompanyCashAndInvestments
Namespace Prefix:	mtg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The amount of the interest payment that was deferred pursuant to the optional deferral provision under the note agreement(s).

No definition available.

Name:	mtg_InterestPaymentDeferred
Namespace Prefix:	mtg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The maximum number of shares of common stock, including shares underlying qualifying warrants, that the entity may be required to issue pursuant to the Alternative Payment Mechanism under the note agreement(s).

No definition available.

Name:	mtg_MaximumNumberOfSharesOfCommonStockIssuableUnderAlternativePaymentMechanism
Namespace Prefix:	mtg_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

The maximum number of trading days within a specified time period that the closing sale price of the entity's common stock exceeds the prevailing conversion price of the debentures by a specified percentage.

No definition available.

Name:	mtg_MaximumNumberOfTradingDays
Namespace Prefix:	mtg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

The maximum ratio of total net proceeds of all issuances of qualifying securities to the aggregate principal amount of the debentures. Pursuant to the Alternative Payment Mechanism under the note agreement(s), the entity may not issue qualifying preferred stock if the total net proceeds exceed this ratio.

No definition available.

Name:	mtg_MaximumPercentageOfAggregatePrincipalAmountOfDebentures
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The maximum length of time for which interest payments on the convertible debentures may be deferred without giving rise to an event of default.

No definition available.

Name:	mtg_MaximumPeriodForWhichInterestPaymentsMayBeDeferredWithoutGivingRiseToEventOfDefault
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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

The minimum number of consecutive interest periods for which interest payments may be deferred pursuant to an optional deferral provision under the note agreement(s).

No definition available.

Name:	mtg_MinimumNumberOfConsecutiveInterestPeriodsForWhichInterestPaymentsMayBeDeferred
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

The minimum number of trading days within a specified time period that the closing sale price of the entity's common stock exceeds the prevailing conversion price of the debentures by a specified percentage.

No definition available.

Name:	mtg_MinimumNumberOfTradingDays
Namespace Prefix:	mtg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

The minimum percentage of consolidated shareholders' equity that a subsidiary's shareholders' equity must represent to be deemed a designated subsidiary as defined by the note agreements.

No definition available.

Name:	mtg_MinimumPercentageOfConsolidatedShareholdersEquityThatDeterminesDesignatedSubsidiaryStatus
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The modified duration of the holding company's investment portfolio, excluding cash and cash equivalents, at balance sheet date.

No definition available.

Name:	mtg_ModifiedDurationOfHoldingCompanyInvestments
Namespace Prefix:	mtg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

The percentage calculated as the average closing price of the entity's common stock times the number of the entity's outstanding shares of common stock. The average price is determined over a specified period ending before the issuance of the common stock or warrants being sold, and the number of outstanding shares is determined as of the date of the entity's most recent publicly released financial statements.

No definition available.

Name:	mtg_NetProceedsCap
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The ratio of the closing sale price of the entity's common stock over the prevailing conversion price of the debentures as of the redemption date.

No definition available.

Name:	mtg_PercentageOfConversionPrice
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The maximum period, in business days, after payment of interest on debentures that was not deferred, in which the entity must begin reasonable commercial efforts to sell qualifying securities to unaffiliated persons pursuant to the optional deferral provision under the note agreement(s).

No definition available.

Name:	mtg_PeriodInWhichReasonableCommercialEffortsMustBeginMaximum
Namespace Prefix:	mtg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

The minimum period in which the entity must begin reasonable commercial efforts to sell qualifying securities to unaffiliated persons pursuant to the optional deferral provision under the note agreement(s).

No definition available.

Name:	mtg_PeriodInWhichReasonableCommercialEffortsMustBeginMinimum
Namespace Prefix:	mtg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The period immediately prior to an election to convert debentures during which the average price of shares traded is determined for the purpose of calculating the conversion rate.

No definition available.

Name:	mtg_PeriodPrecedingElectionToConvert
Namespace Prefix:	mtg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

The principal amount of notes used in the determination of the initial conversion rate, which is subject to adjustment.

No definition available.

Name:	mtg_PrincipalAmountOfNotesUsedInDeterminingConversionRate
Namespace Prefix:	mtg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The amount of unrealized gains or losses on investments held at our holding company at balance sheet date.

No definition available.

Name:	mtg_UnrealizedGainLossOnHoldingCompanyInvestments
Namespace Prefix:	mtg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The price per share of the conversion feature embedded in the debt instrument.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 20

-Section 50

-Paragraph 5

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6928298&loc=SL6031898-161870

Name:	us-gaap_DebtInstrumentConvertibleConversionPrice1
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Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

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Name:	us-gaap_DebtInstrumentFaceAmount
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Topic 825

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Period Type:	instant

Contractual interest rate for funds borrowed, under the debt agreement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

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Period Type:	instant

Name:	us-gaap_DebtInstrumentLineItems
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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Name Regulation S-X (SX)

-Number 210

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Amount of the required periodic payments applied to interest.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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Fair value of subordinated debt obligations including, but not limited to, subordinated loan, subordinated bond, subordinated debenture or junior debt.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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Data Type:	xbrli:monetaryItemType
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The amount of cash paid for interest during the period.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

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-Topic 230

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Name:	us-gaap_InterestPaid
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Name:	us-gaap_LiabilitiesFairValueDisclosureAbstract
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Fair value portion of notes payable.

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-Name Regulation S-X (SX)

-Number 210

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-Paragraph 19, 20, 22

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-Topic 210

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-Subparagraph (SX 210.5-02.19,20,22)

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Name:	us-gaap_NotesPayableFairValueDisclosure
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The cash inflow from the issuance of a long-term debt instrument which can be exchanged for a specified amount of another security, typically the entity's common stock, at the option of the issuer or the holder.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

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Name:	us-gaap_ProceedsFromConvertibleDebt
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

No definition available.

Name:	us-gaap_RepaymentsOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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Name:	us-gaap_LongtermDebtTypeAxis=mtg_ConvertibleSeniorNotesDueApril2020Member
Namespace Prefix:
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Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_ConvertibleDebtMember
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