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Litigation and Contingencies (Details) (USD $)
 6 Months Ended 12 Months Ended 36 Months Ended
Jun. 30, 2009
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Dec. 31, 2011
Class action complaint under RESPA [Abstract]                
Civil penalty     $ 2,650,000mtg_LossContingencyCivilPenalty          
Period of existing captive reinsurance agreement   10 years 10 years          
Curtailments [Abstract]                
Average paid claim reduction due to curtailments (in hundredths)   6.70%mtg_AveragePaidClaimsReductionDueToCurtailments 5.80%mtg_AveragePaidClaimsReductionDueToCurtailments          
Number of days after claim paid within which objection must be received for review   90 days            
Claims resolved by rescissions [Abstract]                
Mitigation of paid losses by rescission of policies   97,000,000mtg_MitigationOfPaidLossesByRescissionOfPolicies 135,000,000mtg_MitigationOfPaidLossesByRescissionOfPolicies 300,000,000mtg_MitigationOfPaidLossesByRescissionOfPolicies       3,000,000,000mtg_MitigationOfPaidLossesByRescissionOfPolicies
Percentage of claims received in a quarter resolved by rescission, lower range limit (in hundredths)   5.00%mtg_PercentageOfClaimsReceivedInQuarterResolvedByRescissionLowerRangeLimit            
Percentage of claims received in a quarter resolved by rescission, upper range limit (in hundredths) 28.00%mtg_PercentageOfClaimsReceivedInQuarterResolvedByRescissionUpperRangeLimit              
Statute of limitations to bring legal proceedings disputing right to rescind coverage   3 years            
Statute of limitations to bring legal proceedings after notice of rescission   2 years            
Mitigation of incurred losses by rescission of policies   0mtg_MitigationOfIncurredLossesByRescissionOfPolicies 0mtg_MitigationOfIncurredLossesByRescissionOfPolicies 0mtg_MitigationOfIncurredLossesByRescissionOfPolicies 0mtg_MitigationOfIncurredLossesByRescissionOfPolicies 200,000,000mtg_MitigationOfIncurredLossesByRescissionOfPolicies 2,500,000,000mtg_MitigationOfIncurredLossesByRescissionOfPolicies  
Reduction in estimated rescissions       200,000,000mtg_ReductionInEstimatedRescissions        
Maximum exposure above estimate provision for claims paying practices   626,000,000mtg_MaximumExposureAboveEstimateProvision            
Exposure from rescission practices (in hundredths)   60.00%mtg_ExposureFromClaimsPayingPracticesRelatedToCountrywideMatter            
Maximum exposure associated with other discussions and legal proceedings   16,000,000mtg_MaximumExposureAssociatedWithOtherDiscussionsLegalProceedings            
Other Legal Proceedings [Abstract]                
Underwriting remedy expense   4,000,000mtg_UnderwritingRemedyExpense 5,000,000mtg_UnderwritingRemedyExpense 27,000,000mtg_UnderwritingRemedyExpense        
Class Action Complaint Under RESPA [Member] | Pending Litigation [Member]                
Class action complaint under RESPA [Abstract]                
Number of lawsuits   12mtg_LossContingencyNumberOfLawsuits
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= mtg_ClassActionComplaintUnderRespaMember		            
Number of cases dismissed prior to February 2015   7mtg_LossContingencyClaimsDismissedNumberPriorToFebruary2015
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= mtg_ClassActionComplaintUnderRespaMember		            
Number of cases dismissed or expected to be dismissed   5mtg_LossContingencyClaimsDismissedOrExpectedToBeDismissedNumber
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= mtg_ClassActionComplaintUnderRespaMember		            
Countrywide Dispute [Member] | Pending Litigation [Member]                
Claims resolved by rescissions [Abstract]                
Amount of damages sought   $ 700,000,000us-gaap_LossContingencyDamagesSoughtValue
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= mtg_CountrywideDisputeMember		            
Lawsuits Alleging Improper Recording and Foreclosure Activities by MERS [Member] | Pending Litigation [Member]                
Lawsuits alleging improper recording and foreclosure activities by MERS [Abstract]                
Number of lawsuits naming non-insurance subsidiary as defendant   8mtg_NumberOfLawsuitsNamingNonInsuranceSubsidiaryAsDefendant
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= mtg_LawsuitsAllegingImproperRecordingAndForeclosureActivitiesByMersMember		            
Number of lawsuits naming subsidiary as defendant that were dismissed   7mtg_NumberOfLawsuitsNamingSubsidiaryAsDefendantThatWereDismissed
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= mtg_LawsuitsAllegingImproperRecordingAndForeclosureActivitiesByMersMember