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Note 1 - Condensed Consolidated Financial Statements - Business Overview, Recent Developments and Significant Accounting Policies Business Overview (Details) $ in Billions | 9 Months Ended
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Sep. 30, 2018 USD ($) segment
Business Overview [Abstract] |
Number of Reportable Business Segments | segment | 2
Mortgage Insurance Segment |
Mortgage Insurance [Abstract] |
Private Mortgage Insurance Protects Lenders For Loans Made With Less Than This Maximum Down Payment Percentage | 20.00%
Private Mortgage Insurance Protects Lenders For Refinancings Made to Home Buyers With Less Than This Maximum Equity-Ownership Percentage | 20.00%
Mortgage Insurance Segment | Total primary reserves |
Mortgage Insurance [Abstract] |
Direct Primary Mortgage Insurance RIF | $ | $ 55.6

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- Definition Business Overview [Abstract] + References No definition available. + Details
Name: | rdn_BusinessOverviewAbstract
Namespace Prefix: | rdn_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Mortgage Insurance [Abstract] + References No definition available. + Details
Name: | rdn_MortgageInsuranceAbstract
Namespace Prefix: | rdn_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition This element represents the maximum down payment percentage on a home's purchase price that is used to measure the need for private mortgage insurance. + References No definition available. + Details
Name: | rdn_PrivateMortgageInsuranceProtectsLendersForLoansMadeWithLessThanThisMaximumDownPaymentPercentage
Namespace Prefix: | rdn_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition This element represents the maximum equity-ownership percentage on a home's value that is used to measure the need for private mortgage insurance for refinancings. + References No definition available. + Details
Name: | rdn_PrivateMortgageInsuranceProtectsLendersForRefinancingsMadetoHomeBuyersWithLessThanThisMaximumEquityOwnershipPercentage
Namespace Prefix: | rdn_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Risk In Force + References No definition available. + Details
Name: | rdn_RiskInForce
Namespace Prefix: | rdn_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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

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