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Litigation and Contingencies (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract] |  |
Average paid claim reduction due to curtailments (in hundredths) | 3.60% | 5.00%
Maximum exposure to loss | $ 40 |

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- Definition Average paid claim reduction due to curtailments. + References No definition available. + Details
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- Definition Estimated maximum exposure associated with other discussions and legal proceedings related to claims paying practices. + References No definition available. + Details
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Namespace Prefix: | mtg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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