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Loss Reserves - Aging of Primary Default Inventory and Number of Delinquent Payments (Details) - loan | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
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Aging of the Primary Default Inventory [Abstract] |  |  |  |
3 months or less | 9,447 | 9,829 | 17,119 |
4 - 11 months | 9,664 | 9,655 | 12,050 |
12 months or more | 10,917 | 13,414 | 17,387 |
Total primary default inventory | 30,028 | 32,898 | 46,556 | 50,282
3 months or less (in hundredths) | 32.00% | 30.00% | 37.00% |
4 - 11 months (in hundredths) | 32.00% | 29.00% | 26.00% |
12 months or more (in hundredths) | 36.00% | 41.00% | 37.00% |
Total primary default inventory (in hundredths) | 100.00% | 100.00% | 100.00% |
Primary claims received inventory included in ending default inventory | 538 | 809 | 954 |

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- References No definition available. + Details
Name: | mtg_AgingOfPrimaryDefaultInventoryAbstract
Namespace Prefix: | mtg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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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

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- Definition The number of loans in the primary default inventory that are in the claims received inventory. + References No definition available. + Details
Name: | mtg_PrimaryDefaultInventoryClaimsReceivedInventory
Namespace Prefix: | mtg_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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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
Name: | mtg_PrimaryDefaultInventoryFourToElevenMonths
Namespace Prefix: | mtg_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- 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
Name: | mtg_PrimaryDefaultInventoryFourToElevenMonthsPercentage
Namespace Prefix: | mtg_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The percentage of loans in the primary default inventory. + References No definition available. + Details
Name: | mtg_PrimaryDefaultInventoryPercentage
Namespace Prefix: | mtg_
Data Type: | num:percentItemType
Balance Type: | na
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
Name: | mtg_PrimaryDefaultInventoryThreeMonthsOrLess
Namespace Prefix: | mtg_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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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
Name: | mtg_PrimaryDefaultInventoryThreeMonthsOrLessPercentage
Namespace Prefix: | mtg_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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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
Name: | mtg_PrimaryDefaultInventoryTwelveMonthsOrMore
Namespace Prefix: | mtg_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- 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
Name: | mtg_PrimaryDefaultInventoryTwelveMonthsOrMorePercentage
Namespace Prefix: | mtg_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant