Amortization Period Excess of Loss Reinsurance Coverage

No definition available.

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

- Definition

The percent of delinquent reinsured principal balance of loans to total reinsured principal balance of loans that causes a trigger event if exceeded.

+ References

No definition available.

+ Details

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

- Definition

Reinsurance, Quota Share Reinsurance Agreement, Terms, Annual Loss Ratio Cap

+ References

No definition available.

+ Details

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

- Definition

Reinsurance, Quota Share Reinsurance Agreement, Terms, Ceding Commission, Percent

+ References

No definition available.

+ Details

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

- Definition

Reinsurance, Quota Share Reinsurance Agreement, Terms, Contingent Termination Fee

+ References

No definition available.

+ Details

Name:	mtg_ReinsuranceQuotaShareReinsuranceAgreementTermsContingentTerminationFee
Namespace Prefix:	mtg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Reinsurance, Quota Share Reinsurance Agreement, Terms, Lifetime Loss Ratio Cap

+ References

No definition available.

+ Details

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

- Definition

Reinsurance, Quota Share Reinsurance Agreement, Terms, Quota Share, Percent

+ References

No definition available.

+ Details

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

- Definition

Reinsurance, Quota Share Reinsurance Agreement, Terms, Threshold For Private Mortgage Insurer Eligibility Requirements For Termination Election

+ References

No definition available.

+ Details

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

- Definition

Reinsurance Retention Policy, Debt Issued To Cover Insurance, Term

+ References

No definition available.

+ Details

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

- Definition

Reinsurance Retention Policy, Percent Of Total Trust Assets Invested In Cash Or Direct U.S. Federal Government Obligations

+ References

No definition available.

+ Details

Name:	mtg_ReinsuranceRetentionPolicyPercentOfTotalTrustAssetsInvestedInCashOrDirectU.S.FederalGovernmentObligations
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Reinsurance Retention Policy, Remaining Excess of Loss Reinsurance Coverages

+ References

No definition available.

+ Details

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

- Definition

Amount of earned premiums ceded to other entities.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 2
-Subparagraph (SX 210.12-17(Column C))
-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(1))
-URI http://asc.fasb.org/extlink&oid=116637232&loc=SL114874131-224263

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=84167274&loc=d3e27175-158546

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

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

- Definition

Ratio of non-life losses and loss expenses to non-life net premiums earned.

+ References

No definition available.

+ Details

Name:	us-gaap_LossRatio
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387

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

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (cc)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

+ Details

Name:	us-gaap_ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=mtg_QuotaShareReinsuranceAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=mtg_QuotaShareReinsuranceAgreement2021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=mtg_CreditUnionQuotaShareReinsuranceAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=mtg_QuotaShareReinsuranceAgreement2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=mtg_ExcessofLossReinsuranceAgreementsHomeReEntitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=mtg_FinancialAssetEqualToOrGreaterThan60DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Reinsurance - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended		12 Months Ended
Reinsurance - Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 28, 2021	Dec. 31, 2017
Effects of Reinsurance [Line Items]
Reinsurance recoverable on loss reserves		$ 21,641				$ 95,042	$ 21,641	$ 33,328		$ 48,474
Ceded premiums earned						$ 188,729	129,337	111,391
Quota Share Reinsurance Agreements
Effects of Reinsurance [Line Items]
Ceding commission, percentage						20.00%
Annual loss ratio cap						300.00%
Lifetime loss ratio cap						200.00%
Threshold for private mortgage insurer eligibility requirements for termination election						90.00%
Reinsurance recoverable on loss reserves		$ 21,600				$ 95,000	21,600
Quota Share Reinsurance Agreement, 2021 \| Forecast
Effects of Reinsurance [Line Items]
Quota Share %				15.00%	12.50%
Annual loss ratio					57.50%
Credit Union QSR
Effects of Reinsurance [Line Items]
Quota Share %						65.00%
Annual loss ratio						50.00%
Threshold for private mortgage insurer eligibility requirements for termination election						80.00%
2015 QSR
Effects of Reinsurance [Line Items]
Quota Share %		15.00%	30.00%			15.00%
Annual loss ratio						68.00%
Contingent termination fee	$ 6,800
Excess of Loss Reinsurance Agreements, Home Re Entities
Effects of Reinsurance [Line Items]
Amortization period excess of loss reinsurance coverage						10 years
Ceded premiums earned						$ 20,800	$ 17,600	$ 2,800
Excess of Loss Reinsurance Agreements, Home Re Entities \| Financial Asset Equal To Or Greater Than 60 Days Past Due
Effects of Reinsurance [Line Items]
Percent of delinquent reinsured loans to total reinsured loans						4.00%
Home Re special purpose insurers
Effects of Reinsurance [Line Items]
Debt issued to cover insurance, term						10 years
Percent of total trust assets invested in cash or direct U.S. federal government obligations						99.50%
Home Re special purpose insurers \| Subsequent Event
Effects of Reinsurance [Line Items]
Remaining Excess of Loss Reinsurance Coverage									$ 398,800