SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - SUPPLEMENTARY NOTES (Details) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Insurance Subsidiaries
Supplementary Insurance Information, by Segment [Line Items]
Percentage of statutory policyholders surplus used to determine maximum allowable dividends	10.00%
Adjusted statutory net income measurement period	3 years
Adjusted statutory net income dividend payment measurement period	2 years
Distributions from other insurance subsidiaries to the parent company	$ 0	$ 0	$ 52,001,000
Proceeds from contribution from parent	$ 0	0
Convertible Junior Subordinated Debentures, at 9% per annum, Due 2063
Supplementary Insurance Information, by Segment [Line Items]
Stated interest rate (in hundredths)	9.00%
Mortgage Guaranty Insurance Corporation
Supplementary Insurance Information, by Segment [Line Items]
Dividends paid to the parent company	$ 220,000,000	140,000,000	64,000,000
Additions to the surplus of MGIC from parent company funds	0	$ 0	$ 36,025,000
Mortgage Guaranty Insurance Corporation \| Convertible Junior Subordinated Debentures, at 9% per annum, Due 2063
Supplementary Insurance Information, by Segment [Line Items]
Investment in holding company debt	$ 132,700,000

Cash Distributions Paid To Parent Company

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The investment in holding company debt at the end of the period.

+ References

No definition available.

+ Details

Name:	mtg_InvestmentInHoldingCompanyDebt
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The percentage of statutory policyholders' surplus, as defined, as of the preceding calendar year end which is used in the determination of the maximum allowable dividends that the insurance subsidiaries may pay in any twelve-month period without regulatory approval by the Office of the Commissioner of Insurance of the State of Wisconsin.

+ References

No definition available.

+ Details

Name:	mtg_PercentageOfStatutoryPolicyholdersSurplusUsedToDetermineMaximumAllowableDividends
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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Statutory Accounting Practices, Statutory Amount Available For Dividend Payments Without Regulatory Approval, Measurement, Measurement Period

+ References

No definition available.

+ Details

Name:	mtg_StatutoryAccountingPracticesStatutoryAmountAvailableForDividendPaymentsWithoutRegulatoryApprovalMeasurementMeasurementPeriod
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- Definition

Statutory Accounting Practices, Statutory Amount Available for Dividend Payments without Regulatory Approval, Measurement, Dividend Payment Period

+ References

No definition available.

+ Details

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- Definition

Additions to the surplus of a subsidiary from parent company funds.

+ References

No definition available.

+ Details

Name:	mtg_SurplusContributionFromParent
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents disclosure of the total aggregate cash dividends paid to the entity by consolidated subsidiaries, by unconsolidated subsidiaries, and by 50% or less owned persons accounted for using the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04(b))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 235
-Section S99
-Paragraph 2
-Subparagraph (SX 210.9-06)
-URI http://asc.fasb.org/extlink&oid=6882215&loc=d3e537971-122884

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Balance Type:	na
Period Type:	instant

- Definition

The cash inflow from parent as a source of financing that is recorded as additional paid in capital.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3255-108585

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Balance Type:	debit
Period Type:	duration

- 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

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Period Type:	duration

- Details

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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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