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Loss Reserves - Delinquent Items (Details) - loan | Sep. 30, 2020 | Jun. 30, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Dec. 31, 2018
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Insurance Loss Reserves [Abstract] |  |  |  |  |  |
Percent of delinquency inventory subject to a Covid-19 forbearance plan (as a percent) | 67.00% |  |  |  |  |
Aging of the Primary Delinquent Inventory [Abstract] |  |  |  |  |  |
3 months or less (in loans) | 15,879 |  | 9,447 | 9,462 |  |
3 months or less (as a percent) | 25.00% |  | 32.00% | 32.00% |  |
4 - 11 months (in loans) | 37,702 |  | 9,664 | 9,082 |  |
4 - 11 months (as a percent) | 58.00% |  | 32.00% | 30.00% |  |
12 months or more (in loans) | 10,837 |  | 10,917 | 11,396 |  |
12 months or more (as a percent) | 17.00% |  | 36.00% | 38.00% |  |
Total primary delinquent inventory (in loans) | 64,418 | 69,326 | 30,028 | 29,940 | 29,795 | 32,898
Total primary delinquent inventory (as a percent) | 100.00% |  | 100.00% | 100.00% |  |
Primary claims received inventory included in ending delinquent inventory (in loans) | 172 |  | 538 | 557 |  |
Percent of 12 months or more delinquent inventory, delinquent for more than 36 months (as a percent) | 32.00% |  | 36.00% | 36.00% |  |

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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
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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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Period Type: | instant

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- Definition The percentage of loans in the primary default inventory. + References No definition available. + Details
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- Definition Primary Default Inventory, Percent of Default Inventory Subject To Covid-19 Forbearance Plan + References No definition available. + Details
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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
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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 three months or less 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 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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- 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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- References No definition available. + Details
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