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Loss Reserves - Aging of Primary Default Inventory (Details) - loan | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
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Aging of the Primary Default Inventory [Abstract] |  |  |  |
3 months or less | 11,542 | 9,447 | 9,829 |
4 - 11 months | 34,620 | 9,664 | 9,655 |
12 months or more | 11,548 | 10,917 | 13,414 |
Total primary default inventory | 57,710 | 30,028 | 32,898 | 46,556
3 months or less (in hundredths) | 20.00% | 32.00% | 30.00% |
4 - 11 months (in hundredths) | 60.00% | 32.00% | 29.00% |
12 months or more (in hundredths) | 20.00% | 36.00% | 41.00% |
Total primary default inventory (in hundredths) | 100.00% | 100.00% | 100.00% |
Primary claims received inventory included in ending default inventory | 159 | 538 | 809 |
Percent of inventory in default for more than 36 consecutive months | 31.00% | 36.00% | 38.00% |

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

X
- 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

X
- 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

X
- 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

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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
Name: | mtg_PrimaryDefaultInventoryTwelveMonthsOrMorePercentOfPrimaryDefaultInventoryInDefaultForTwelveMonthsOrMoreInDefaultForMoreThanThirtySixConsecutiveMonths
Namespace Prefix: | mtg_
Data Type: | num:percentItemType
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