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Note 10 - Losses and Loss Adjustment Expense Reserve Activity (Details) - incident | Mar. 31, 2019 | Mar. 31, 2018
--- | --- | ---
Number of Natural Disasters | 2 |
Mortgage Insurance Segment |  |
Default To Claim Rate Estimate, Gross, For New Defaults |  | 9.50%
Primary Mortgage Product | Mortgage Insurance Segment |  |
Default To Claim Rate Estimate, Gross, For New Defaults | 8.00% |
Hurricanes Harvey and Irma | Primary Mortgage Product | Mortgage Insurance Segment |  |
Default To Claim Rate Estimate, Gross, For New Defaults | 3.00% |

X
- Definition This concept represents the rate at which newly reported defaulted loans are expected to result in a claim. + References No definition available. + Details
Name: | rdn_DefaultToClaimRateEstimateGrossForNewDefaults
Namespace Prefix: | rdn_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Number of natural disasters + References No definition available. + Details
Name: | rdn_NumberofNaturalDisasters
Namespace Prefix: | rdn_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=rdn_MortgageInsuranceSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ProductOrServiceAxis=rdn_PrimaryMortgageProductMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ConcentrationRiskByTypeAxis=rdn_HurricanesHarveyandIrmaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |