Significant Accounting Policies - Additional Information (Details)	12 Months Ended
Dec. 31, 2023 USD ($) loan	Dec. 31, 2022 USD ($)
Significant Accounting Policies [Line Items]
Minimum number of payments past due to be in default \| loan	2
Related party transaction amount \| $	$ 0	$ 0
Minimum
Significant Accounting Policies [Line Items]
Award vesting period	1 year
Maximum
Significant Accounting Policies [Line Items]
Award vesting period	3 years

The number of payments past due in order for such loans to be considered in default.

No definition available.

Name:	mtg_MinimumNumberOfPaymentsPastDueToBeInDefault
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Significant Accounting Policies

+ References

No definition available.

+ Details

Name:	mtg_SignificantAccountingPoliciesLineItems
Namespace Prefix:	mtg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of transactions with related party during the financial reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483326/850-10-50-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483326/850-10-50-3

+ Details

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

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: