Subsequent Events Subsequent Events (Details) - Subsequent event	Oct. 30, 2018 USD ($)
Subsequent Event [Line Items]
Aggregate excess of loss reinsurance coverage	$ 318,600,000
Amortization period excess of loss reinsurance coverage	10 years
Mortgage Guaranty Insurance Corporation
Subsequent Event [Line Items]
Aggregate excess of loss reinsurance retained by company	$ 168,700,000
Excess of loss reinsurance coverage, option to terminate period	7 years
Home Re \| Mortgage insurance-linked notes
Subsequent Event [Line Items]
Debt instrument, face amount	$ 318,600,000.0
Term of mortgage insurance-linked notes	10 years

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

Reinsurance Retention Policy, Option To Terminate, Period

+ References

No definition available.

+ Details

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

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Stated amount that is the upper retention limit per entity policy.

+ References

No definition available.

+ Details

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

- Definition

Amount of risk reinsured in excess of stated retention per company policy.

+ References

No definition available.

+ Details

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

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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Period Type:	duration

- Details

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

- Details

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

- Details

Name:	dei_LegalEntityAxis=mtg_HomeRe20181Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=mtg_MortgageInsuranceLinkedNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: