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Loss Reserves - Aging of Primary Default Inventory (Details) - loan | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
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Aging of the Primary Default Inventory [Abstract] |  |  |  |
3 months or less | 7,586 | 11,542 | 9,447 |
4 - 11 months | 7,990 | 34,620 | 9,664 |
12 months or more | 17,714 | 11,548 | 10,917 |
Total primary default inventory | 33,290 | 57,710 | 30,028 | 32,898
3 months or less (in hundredths) | 23.00% | 20.00% | 32.00% |
4 - 11 months (in hundredths) | 24.00% | 60.00% | 32.00% |
12 months or more (in hundredths) | 53.00% | 20.00% | 36.00% |
Total primary default inventory (in hundredths) | 100.00% | 100.00% | 100.00% |
Primary claims received inventory included in ending default inventory | 211 | 159 | 538 |
Percent of inventory in default for more than 36 consecutive months | 20.00% | 31.00% | 36.00% |

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- References No definition available. + Details
Name: | mtg_AgingOfPrimaryDefaultInventoryAbstract
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- Definition The number of loans in the primary default inventory. + References No definition available. + Details
Name: | mtg_PrimaryDefaultInventory
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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
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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_
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- Definition The percentage of loans in the primary default inventory. + References No definition available. + Details
Name: | mtg_PrimaryDefaultInventoryPercentage
Namespace Prefix: | mtg_
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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
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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: | dtr-types:percentItemType
Balance Type: | na
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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
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- Definition Primary Default Inventory Twelve Months Or More, Percent Of Primary Default Inventory In Default For Twelve Months Or More In Default For More Than Thirty Six Consecutive Months + References No definition available. + Details
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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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Namespace Prefix: | mtg_
Data Type: | dtr-types:percentItemType
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