Name:	mtg_PercentageOfPriorYearDefaultInventoryResolved
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
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Period Type:	duration

The number of loans included in the pool insurance notice inventory as of the end of the reporting period.

No definition available.

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

The carrying amount for the estimate of premiums to be refunded on expected claim payments.

No definition available.

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

The number of loans in the primary default inventory that are in FEMA Individual Assistance Disaster Areas.

No definition available.

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

The number of new notices reported in locations that FEMA has declared as Individual Assistance Disaster Areas (IADAs).

No definition available.

Name:	mtg_PrimaryDefaultInventoryNewNoticesInFEMAIndividualAssistanceDisasterAreas
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
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Period Type:	duration

Other deductions from the primary default inventory, including items removed for settlements

No definition available.

Name:	mtg_PrimaryDefaultInventoryOtherItemsRemoved
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Primary Default Inventory Twelve Months Or More, Percent Of Primary Default Inventory In Default For Twelve Months Or More In Default For More Than Thirty Six Consecutive Months

No definition available.

Name:	mtg_PrimaryDefaultInventoryTwelveMonthsOrMorePercentOfPrimaryDefaultInventoryInDefaultForTwelveMonthsOrMoreInDefaultForMoreThanThirtySixConsecutiveMonths
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
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Period Type:	instant

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.

No definition available.

Name:	us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseLineItems
Namespace Prefix:	us-gaap_
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Amount, after effects of reinsurance, of payments to settle claims incurred in prior periods and related claims settlement costs.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

Name:	us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

Name:	us-gaap_NoncatastrophicEventAxis=mtg_IncreaseDecreaseInExpectedClaimRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_NoncatastrophicEventAxis=mtg_SettlementsForCommutationsOfCoveragePoolsOfNonperformingLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_NoncatastrophicEventAxis=mtg_SettlementsForCommutationsOfCoveragePoolsOfNonperformingLoansAndClaimsPayingPracticesMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Name:	us-gaap_NoncatastrophicEventAxis=mtg_SettlementsForClaimsPayingPracticesAndNonperformingLoanSaleMember
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Data Type:	na
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