Reinsurance Reinsurance - Variable Interest Entity (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Home Re special purpose insurers
Effects of Reinsurance [Line Items]
Term of mortgage insurance-linked notes	10 years
Remaining First Layer Retention	$ 354,421
Remaining Excess of Loss Reinsurance Coverages	634,375
Home Re 2018-1
Effects of Reinsurance [Line Items]
Remaining First Layer Retention	168,691
Remaining Excess of Loss Reinsurance Coverages	318,636
Total VIE Assets	318,636	$ 318,636
Home Re 2019-1
Effects of Reinsurance [Line Items]
Remaining First Layer Retention	185,730
Remaining Excess of Loss Reinsurance Coverages	315,739
Total VIE Assets	$ 315,739

Reinsurance Retention Policy, Debt Issued To Cover Insurance, Term

No definition available.

Name:	mtg_ReinsuranceRetentionPolicyDebtIssuedToCoverInsuranceTerm
Namespace Prefix:	mtg_
Data Type:	xbrli:durationItemType
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

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

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

Carrying amount of the assets in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph a
-URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5728-111685

+ Details

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

- Details

Name:	dei_LegalEntityAxis=mtg_HomeRespecialpurposeinsurersMember
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:	dei_LegalEntityAxis=mtg_HomeRe20191Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: