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v3.10.0.1

Subsequent Events Subsequent Events (Details) - Subsequent event | Oct. 30, 2018 USD ($)
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Subsequent Event [Line Items] |
Aggregate excess of loss reinsurance coverage | $ 318,600,000
Amortization period excess of loss reinsurance coverage | 10 years
Mortgage Guaranty Insurance Corporation |
Subsequent Event [Line Items] |
Aggregate excess of loss reinsurance retained by company | $ 168,700,000
Excess of loss reinsurance coverage, option to terminate period | 7 years
Home Re | Mortgage insurance-linked notes |
Subsequent Event [Line Items] |
Debt instrument, face amount | $ 318,600,000.0
Term of mortgage insurance-linked notes | 10 years

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- Definition Amortization Period Excess of Loss Reinsurance Coverage + References No definition available. + Details
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Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Reinsurance Retention Policy, Option To Terminate, Period + References No definition available. + Details
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Namespace Prefix: | mtg_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399 + Details
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Period Type: | instant

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- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
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- Definition Stated amount that is the upper retention limit per entity policy. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of risk reinsured in excess of stated retention per company policy. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
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Period Type: | duration

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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |