Basis of presentation (Details) (USD $)	6 Months Ended	9 Months Ended	12 Months Ended
Jun. 30, 2009	Sep. 30, 2011	Dec. 31, 2010	Dec. 31, 2009
Basis of presentation (Details) [Abstract]
Number of jurisdictions with risk-to-capital requirements		16
Maximum permitted risk-to-capital ratio commonly applied		25 to 1
Risk-to-capital ratio at end of period		22.2 to 1	19.8 to 1
Excess of policyholders position over required regulatory minimum		$ 50,000,000	$ 225,000,000
Risk-to-capital ratio of combined insurance operations, including reinsurance affiliates, at end of period		24.0 to 1	23.2 to 1
Statutory deferred tax assets		133,000,000
Percentage of new insurance written in jurisdictions with risk-to-capital requirements (in hundreths)		50.00%	50.00%
Contribution to subsidiary under GSE agreement		200,000,000
Minimum number of jurisdictions that could prevent the entity from continuing to write new insurance		1
Claim payments to be paid by PMI Mortgage Insurance Company (in hundredths)		50.00%
Mitigation of paid losses by rescission of policies		500,000,000	1,200,000,000	1,200,000,000
Estimated mitigation of incurred losses by impact of rescissions on loss reserving methodology			$ 200,000,000	$ 2,500,000,000
Percentage of claims received in a quarter resolved by rescission, lower range limit (in hundredths)		18.00%
Percentage of claims received in a quarter resolved by rescission, upper range limit (in hundredths)	28.00%	21.00%
Number of loans in primary delinquency inventory		180,894
Period during which the insured may initiate legal proceedings (in years)		3

The percentage of claim payments to be paid by the company's competitor pursuant to the order of the insurance department related to a partial claim payment plan.

Name:	mtg_ClaimPaymentsToBePaidByPmiMortgageInsuranceCompany
Namespace Prefix:	mtg
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Balance Type:	na
Period Type:	instant

- Definition

The amount of the contribution to MGIC Indemnity Corporation (MIC) under an agreement with the GSE's whereby the GSE's approved MIC as an eligible mortgage insurer for a specified time period.

+ Details

Name:	mtg_ContributionToSubsidiaryUnderGseAgreement
Namespace Prefix:	mtg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount by which the entity's policyholders position (the insurer's net worth or surplus, contingency reserve and a portion of the reserves for unearned premiums) exceeded the required regulatory minimum of the entity's domiciliary state.

+ Details

Name:	mtg_ExcessOfPolicyholdersPositionOverRequiredRegulatoryMinimum
Namespace Prefix:	mtg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The minimum number of jurisdictions, including those that do not have specific regulatory capital requirements applicable to mortgage insurers, whose regulatory actions could prevent the entity from continuing to write new insurance in some or all of the jurisdictions in which MGIC Indemnity Corporation is not an eligible mortgage insurer.

+ Details

Name:	mtg_MinimumNumberOfJurisdictionsThatCouldPreventEntityFromContinuingToWriteNewInsurance
Namespace Prefix:	mtg
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount by which incurred losses are estimated to be mitigated as a result of rescissions of loans.

+ Details

Name:	mtg_MitigationOfIncurredLosses
Namespace Prefix:	mtg
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount by which paid losses have been mitigated by the rescissions of policies.

+ Details

Name:	mtg_MitigationOfPaidLossesByRescissionOfPolicies
Namespace Prefix:	mtg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The number of jurisdictions that require a mortgage insurer to maintain a minimum amount of statutory capital relative to the risk in force (or a similar measure) in order for the mortgage insurer to continue to write new business. This is generally referred to as a risk-to-capital requirement.

+ Details

Name:	mtg_NumberOfJurisdictionsWithRiskToCapitalRequirements
Namespace Prefix:	mtg
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of loans comprising the entity's primary delinquency inventory.

+ Details

Name:	mtg_NumberOfLoansInPrimaryDelinquencyInventory
Namespace Prefix:	mtg
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of claims received in a quarter, at lower range limit, that have been resolved by rescission.

+ Details

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Namespace Prefix:	mtg
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of claims received in a quarter, at upper range limit, that have been resolved by rescission.

+ Details

Name:	mtg_PercentageOfClaimsReceivedInQuarterResolvedByRescissionUpperRangeLimit
Namespace Prefix:	mtg
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of new insurance written during the current period that was written in jurisdictions that have risk-to-capital requirements.

+ Details

Name:	mtg_PercentageOfNewInsuranceWrittenInJurisdictionsWithRiskToCapitalRequirements
Namespace Prefix:	mtg
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The maximum number of years, after the lender has obtained title to the property or the property was sold in an approved sale, in which the insured may initiate legal proceedings against the entity.

+ Details

Name:	mtg_PeriodDuringWhichInsuredMayInitiateLegalProceedings
Namespace Prefix:	mtg
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The entity's risk-to-capital ratio as of the end of the reporting period.

+ Details

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Namespace Prefix:	mtg
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	instant

- Definition

The risk-to-capital ratio of the entity's combined insurance operations, including its reinsurance affiliates, as of the end of the reporting period.

+ Details

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Namespace Prefix:	mtg
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	instant

- Definition

The sum of the tax effects as of the balance sheet date of the amounts of all future tax deductions arising from temporary differences between tax basis and statutory accounting principles basis recognition of assets, liabilities, revenues and expenses.

+ Details

Name:	mtg_StatutoryDeferredTaxAssets
Namespace Prefix:	mtg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

For insurance companies, a description of the minimum regulatory capital requirements imposed by state insurance regulators, and restrictions on dividend payments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 60

-Paragraph 60

-Subparagraph h

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 505

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24708-158529

+ Details

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- Details

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