Statutory Information - Statutory financial information of insurance subsidiaries (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Statutory Capital [Abstract]
Statutory net income	$ 1,112,477	$ 279,145	$ 440,944
Statutory policyholders' surplus	976,756	639,878	924,977
Contingency reserve	$ 4,897,284	$ 5,199,405	$ 4,669,724

Contingency reserve liability recorded under Statutory Accounting Practices to protect against loss during periods of extreme contraction.

No definition available.

+ Details

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

- References

No definition available.

+ Details

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- Definition

The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 944
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477908/944-505-50-1

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Period Type:	instant

- Definition

Amount of net income for the period determined using accounting principles prescribed or permitted by insurance regulators.

+ References

+ Details

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