Reinsurance - Summary of Premiums Earned and Losses Incurred (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Premiums earned:
Direct	$ 283,363	$ 284,636	$ 567,384	$ 570,670
Assumed	3,377	3,047	6,819	5,834
Ceded	(43,212)	(44,872)	(88,031)	(91,678)
Net premiums earned	243,528	242,811	486,172	484,826
Losses incurred:
Direct	(14,196)	(15,706)	(3,217)	(4,583)
Assumed	(46)	(31)	(17)	(27)
Losses incurred, net	(18,272)	(17,691)	(13,717)	(11,245)
Quota Share Reinsurance Transactions
Premiums earned:
Ceded	(26,643)	(27,442)	(55,358)	(57,319)
Losses incurred:
Ceded	(4,030)	(1,954)	(10,483)	(6,635)
Profit commission on reinsurance	27,301	34,809	51,885	66,520
Ceding commission on reinsurance	10,789	12,450	21,449	24,768
Excess of Loss Reinsurance Transactions
Premiums earned:
Ceded	$ (16,569)	$ (17,430)	$ (32,673)	$ (34,359)

Reinsurance Ceding Commission Earned

No definition available.

Name:	mtg_ReinsuranceCedingCommissionEarned
Namespace Prefix:	mtg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Reinsurance, Profit Commission

+ References

No definition available.

+ Details

Name:	mtg_ReinsuranceProfitCommission
Namespace Prefix:	mtg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of earned premiums assumed from other entities.

+ References

+ Details

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

- Definition

Amount of earned premiums ceded to other entities.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 235
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.12-17(Column C))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477965/944-235-S99-2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(1))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 605
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479032/944-605-50-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 605
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478832/944-605-45-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 605
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479032/944-605-50-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 605
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479032/944-605-50-1

+ Details

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

- Definition

Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

+ References

+ Details

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

- Definition

Amount, before effect of policies assumed or ceded, of expense related to provision for policy benefits and costs incurred.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(5))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1

+ Details

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

- Definition

Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

+ References

+ Details

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

- Definition

Amount of policy benefits and costs incurred for policies assumed.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479838/944-20-50-3

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(5))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1

+ Details

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

- Definition

Amount of reduction of provision for policy benefits and costs incurred for policies ceded.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Details

Name:	srt_ProductOrServiceAxis=mtg_QuotaShareReinsuranceTransactionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=mtg_ExcessOfLossReinsuranceTransactionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: