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

The amount of minimum policyholder position (MPP) required by the entity's domiciliary state.

No definition available.

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

The number of jurisdictions that require a mortgage insurer to maintain a minimum amount of statutory capital relative to the risk in force (or a similar measure) in order for the mortgage insurer to continue to write new business. This is generally referred to as a risk-to-capital requirement.

No definition available.

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

The percentage of surplus as regards policyholders allowed for determining the amount of admitted statutory deferred tax assets.

No definition available.

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

The entity's risk-to-capital ratio as of the end of the reporting period.

No definition available.

Name:	mtg_RiskToCapitalRatioAtEndOfPeriod
Namespace Prefix:	mtg_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

The risk-to-capital ratio of the entity's combined insurance operations, including its reinsurance affiliates, as of the end of the reporting period.

No definition available.

Name:	mtg_RiskToCapitalRatioOfCombinedInsuranceOperationsIncludingReinsuranceAffiliatesAtEndOfPeriod
Namespace Prefix:	mtg_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

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

The sum of the tax effects as of the balance sheet date of the admitted amounts of all future tax deductions arising from temporary differences between tax basis and statutory accounting principles basis recognition of assets, liabilities, revenues and expenses.

No definition available.

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

For insurance companies, a description of the minimum regulatory capital requirements imposed by state insurance regulators, and restrictions on dividend payments.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 505

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24708-158529

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

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

Name:	dei_LegalEntityAxis=mtg_MortgageGuarantyInsuranceCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: