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

Number of other mortgage insurers agreeing to similar CFPB settlements.

No definition available.

Name:	mtg_NumberOfOtherMortgageInsurersAgreeingToSimilarCfpbSettlements
Namespace Prefix:	mtg_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

Represents percentage of ceding commission under the structure of reinsurance agreement during the reporting period.

No definition available.

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

Represents percentage of quota share under the structure of reinsurance agreement during the reporting period.

No definition available.

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

Represents period under which an entity will not enter into any new captive reinsurance agreement or reinsure any new loans under existing reinsurance agreements.

No definition available.

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

The known and estimated amount recoverable, related to captive agreements, as of the balance sheet date from reinsurers for claims paid or incurred by the ceding insurer and associated claims settlement expenses, including estimated amounts for claims incurred but not reported, and policy benefits, net of any related valuation allowance.

No definition available.

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

The total fair value of the trust fund assets under the entity's captive agreements.

No definition available.

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

The amount of trust fund assets that were transferred to the entity as a result of captive terminations during the period.

No definition available.