Reinsurance, Quota Share Reinsurance Agreement, Terms, Ceding Commission, Percent

No definition available.

Name:	mtg_ReinsuranceQuotaShareReinsuranceAgreementTermsCedingCommissionPercent
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Reinsurance, Quota Share Reinsurance Agreement, Terms, Contingent Termination Fee

+ References

No definition available.

+ Details

Name:	mtg_ReinsuranceQuotaShareReinsuranceAgreementTermsContingentTerminationFee
Namespace Prefix:	mtg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Reinsurance, Quota Share Reinsurance Agreement, Terms, Quota Share, Percent

+ References

No definition available.

+ Details

Name:	mtg_ReinsuranceQuotaShareReinsuranceAgreementTermsQuotaSharePercent
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Reinsurance, Quota Share Reinsurance Agreement, Terms, Threshold For Private Mortgage Insurer Eligibility Requirements For Termination Election

+ References

No definition available.

+ Details

Name:	mtg_ReinsuranceQuotaShareReinsuranceAgreementTermsThresholdForPrivateMortgageInsurerEligibilityRequirementsForTerminationElection
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
Balance Type:	na
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

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

- Definition

Ratio of non-life losses and loss expenses to non-life net premiums earned.

+ References

No definition available.

+ Details

Name:	us-gaap_LossRatio
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (cc)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489

+ Details

Name:	us-gaap_ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=mtg_QuotaShareReinsuranceAgreement2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=mtg_QuotaShareReinsuranceAgreement2017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=mtg_QuotaShareReinsuranceAgreement2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=mtg_QuotaShareReinsuranceAgreementsExcludingCaptivesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Reinsurance - Narrative (Details) - USD ($)	6 Months Ended
Reinsurance - Narrative (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Effects of Reinsurance [Line Items]
Reinsurance recoverable on loss reserves	$ 37,051,000	$ 48,474,000	$ 44,783,000	$ 50,493,000
Quota Share Reinsurance Agreement, 2018
Effects of Reinsurance [Line Items]
Contingent termination fee	$ 0
Threshold for private mortgage insurer eligibility requirements for termination election (less than) (as a percent)	90.00%
Quota share for all policies covered (as a percent)	30.00%
Ceding commission, percentage (as a percent)	20.00%
Loss ratio threshold for profit commissions (as a percent)	62.00%
Quota Share Reinsurance Agreement, 2017
Effects of Reinsurance [Line Items]
Contingent termination fee	$ 0
Threshold for private mortgage insurer eligibility requirements for termination election (less than) (as a percent)	90.00%
Quota share for all policies covered (as a percent)	30.00%
Ceding commission, percentage (as a percent)	20.00%
Loss ratio threshold for profit commissions (as a percent)	60.00%
Quota Share Reinsurance Agreement, 2015
Effects of Reinsurance [Line Items]
Contingent termination fee	$ 0
Threshold for private mortgage insurer eligibility requirements for termination election (less than) (as a percent)	90.00%
Quota share for all policies covered (as a percent)	30.00%
Ceding commission, percentage (as a percent)	20.00%
Loss ratio threshold for profit commissions (as a percent)	60.00%
Quota Share Reinsurance Agreements, Excluding Captive Agreements
Effects of Reinsurance [Line Items]
Reinsurance recoverable on loss reserves	$ 36,500,000	$ 39,300,000