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

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.

No definition available.

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

Deductions from the primary default inventory resulting from loans in default being brought current.

No definition available.

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

Additions to the primary default inventory resulting from new notices of defaulted loans.

No definition available.

Name:	mtg_PrimaryDefaultInventoryNewNotices
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
Balance Type:	na
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

Deductions from the primary default inventory resulting from claims being paid, including those charged to a deductible or captive.

No definition available.

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

Deductions from the primary default inventory resulting from rescissions and denials.

No definition available.

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

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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

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_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Amount, after effects of reinsurance, of payments to settle claims incurred in prior periods and related claims settlement costs.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

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

Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

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

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

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