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The period after a principal or interest payment is due within which a payment must be made to avoid a default pursuant to the restrictive covenants under the note agreement(s).

No definition available.

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

The gain on repurchase of principal outstanding on a debt instrument.

No definition available.

Name:	mtg_GainOnRepurchaseDebtInstrument
Namespace Prefix:	mtg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

Date when the debt instrument is scheduled to be fully repaid, which may be presented in a variety of ways (year, month and year, day, month and year, quarter, etc.).

No definition available.

Name:	mtg_MaturityDate
Namespace Prefix:	mtg_
Data Type:	invest:dateStringItemType
Balance Type:	na
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
Namespace Prefix:	mtg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

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:	us-types:durationStringItemType
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 notes held by holders, in the aggregate, that would allow such holders the right to accelerate the maturity of a given series of notes pursuant to the restrictive covenants under the note agreement(s).

No definition available.

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

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:	instant

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:	instant

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:	instant

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 percentage of the principal amount of the debentures being redeemed which would equal the redemption price under certain conditions as specified in the note agreement(s).

No definition available.

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

The minimum aggregate amount of other debt that, if maturity were accelerated, would result in a default pursuant to the restrictive covenants under the note agreement(s).

No definition available.

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

The minimum aggregate amount of payments due with respect to a final judgment rendered against the entity or any of its subsidiaries and which is not discharged or stayed within certain time limits that would result in a default pursuant to the restrictive covenants under the note agreement(s).

No definition available.

Name:	mtg_RestrictionOnFinalJudgments
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

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.

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Convertible Security

-URI http://asc.fasb.org/extlink&oid=6509036

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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Including the current and noncurrent portions, the carrying value of convertible subordinated debt, as of the balance sheet date, initially scheduled to be repaid after one year or beyond the normal operating cycle if longer. This form of debt 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, and places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Article 5

-Subsection 20, 22

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX210.5-02.22))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Name:	us-gaap_ConvertibleSubordinatedDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

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-Publisher FASB

-Name FASB Staff Position (FSP)

-Number APB14-1

-Paragraph 32

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

The ratio applied to the debt for purposes of determining the number of shares of the equity security into which the debt will be converted.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21521-112644

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Conversion Rate

-URI http://asc.fasb.org/extlink&oid=6509012

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(5))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_DebtInstrumentConvertibleConversionRatio
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Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

The effective interest rate on the liability component of convertible debt instrument which may be settled in cash upon conversion, including partial cash settlement.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 20

-Section 50

-Paragraph 6

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6928298&loc=SL6036836-161870

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-Publisher FASB

-Name FASB Staff Position (FSP)

-Number APB14-1

-Paragraph 33

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_DebtInstrumentConvertibleEffectiveInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Decrease for amounts repaid on the debt instrument for the period.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(f))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph f

-Article 4

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

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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-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

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-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 21

-Paragraph 16, 20

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13476-108611

Name:	us-gaap_DebtInstrumentFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Interest rate stated in the contractual debt agreement.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

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

Amount of subordinated debt obligations (also known as subordinated loan, subordinated bond, subordinated debenture or junior debt) which has a priority ranking after other debts in a dissolution, measured at fair value.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a),(b)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

Name:	us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The amount of cash paid for interest during the period.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4297-108586

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 29

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 25

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3536-108585

Name:	us-gaap_InterestPaid
Namespace Prefix:	us-gaap_
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Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_LiabilitiesFairValueDisclosureAbstract
Namespace Prefix:	us-gaap_
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. This item represents notes payable as of the balance sheet date.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

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

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682