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Loss Reserves - Aging of Primary Default Inventory and Number of Delinquent Payments (Details) - loan | 3 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Abstract] |  |  |  |
Default notices for loans in IADAs | 9,294 | 1,968 |  |
Primary Default Inventory In FEMA Individual Assistance Disaster Areas | 12,446 | 7,162 |  |
Aging of the Primary Default Inventory [Abstract] |  |  |  |
3 months or less | 17,119 | 12,194 | 13,053 |
3 months or less (in hundredths) | 37.00% | 24.00% | 21.00% |
4 - 11 months | 12,050 | 13,450 | 15,763 |
4 - 11 months (in hundredths) | 26.00% | 27.00% | 25.00% |
12 months or more | 17,387 | 24,638 | 33,817 |
12 months or more (in hundredths) | 37.00% | 49.00% | 54.00% |
Total primary default inventory | 46,556 | 50,282 | 62,633 | 79,901
Total primary default inventory (in hundredths) | 100.00% | 100.00% | 100.00% |
Primary claims received inventory included in ending default inventory | 954 | 1,385 | 2,769 |
Primary claims received inventory included in ending default inventory (in hundredths) | 2.00% | 3.00% | 4.00% |
Number of Payments Delinquent [Abstract] |  |  |  |
3 payments or less | 21,678 | 18,419 | 20,360 |
3 payments or less (in hundredths) | 46.00% | 36.00% | 33.00% |
4 - 11 payments | 12,446 | 12,892 | 15,092 |
4 - 11 payments (in hundredths) | 27.00% | 26.00% | 24.00% |
12 payments or more | 12,432 | 18,971 | 27,181 |
12 payments or more (in hundredths) | 27.00% | 38.00% | 43.00% |
Total primary default inventory | 46,556 | 50,282 | 62,633 | 79,901
Total primary default inventory (in hundredths) | 100.00% | 100.00% | 100.00% |

X
- References No definition available. + Details
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- Definition The number of loans in the primary default inventory that have been delinquent for twelve payments or more. + References No definition available. + Details
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- Definition The percentage of loans in the primary default inventory that have been delinquent for twelve payments or more. + References No definition available. + Details
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Data Type: | num:percentItemType
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- Definition The number of loans in the primary default inventory that have been delinquent for three payments or less. + References No definition available. + Details
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- Definition The percentage of loans in the primary default inventory that have been delinquent for three payments or less. + References No definition available. + Details
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Namespace Prefix: | mtg_
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- Definition The number of loans in the primary default inventory that have been delinquent for at least four payments but no more than eleven payments. + References No definition available. + Details
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Namespace Prefix: | mtg_
Data Type: | xbrli:integerItemType
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- Definition The percentage of loans in the primary default inventory that have been delinquent for at least four payments but no more than eleven payments. + References No definition available. + Details
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Namespace Prefix: | mtg_
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- References No definition available. + Details
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- Definition The number of loans in the primary default inventory. + References No definition available. + Details
Name: | mtg_PrimaryDefaultInventory
Namespace Prefix: | mtg_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition The number of loans in the primary default inventory that are in the claims received inventory. + References No definition available. + Details
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- Definition The percentage of loans in the primary default inventory that are in the claims received inventory. + References No definition available. + Details
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- Definition The number of loans that have been in the primary default inventory for at least four months but no greater than eleven months as of the balance sheet date. + References No definition available. + Details
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- Definition The percentage of loans that have been in the primary default inventory for at least four months but no greater than eleven months as of the balance sheet date. + References No definition available. + Details
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Namespace Prefix: | mtg_
Data Type: | num:percentItemType
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- Definition The number of loans in the primary default inventory that are in FEMA Individual Assistance Disaster Areas. + References No definition available. + Details
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- Definition The number of new notices reported in locations that FEMA has declared as Individual Assistance Disaster Areas (IADAs). + References No definition available. + Details
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- Definition The percentage of loans in the primary default inventory. + References No definition available. + Details
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Data Type: | num:percentItemType
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Period Type: | instant

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- Definition The number of loans that have been in the primary default inventory for three months or less as of the balance sheet date. + References No definition available. + Details
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- Definition The percentage of loans that have been in the primary default inventory for three months or less as of the balance sheet date. + References No definition available. + Details
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Data Type: | num:percentItemType
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- Definition The number of loans that have been in the primary default inventory for twelve months or more as of the balance sheet date. + References No definition available. + Details
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Data Type: | xbrli:integerItemType
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- Definition The percentage of loans that have been in the primary default inventory for twelve months or more as of the balance sheet date. + References No definition available. + Details
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Data Type: | num:percentItemType
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- References No definition available. + Details
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