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Note 1 - Condensed Consolidated Financial Statements - Business Overview and Significant Accounting Policies Business Overview (Details) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2020 USD ($) segment | Dec. 31, 2019 USD ($)
Business Overview [Abstract] |  |
Number of Reportable Business Segments | segment | 2 |
Mortgage 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% |
Direct Primary Mortgage Insurance in Force | $ 241,600.0 |
Direct Primary Mortgage Insurance RIF | 60,900.0 |
Credit Risk Transfer Transactions | Mortgage segment |  |
Mortgage Insurance [Abstract] |  |
Direct Primary Mortgage Insurance RIF | $ 332.8 | $ 275.2

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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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- Definition Amount of participating insurance in force. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 50 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6483535&loc=d3e16392-158451 + Details
Name: | us-gaap_ParticipatingPoliciesAmountInForce
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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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: | rdn_RiskinForceAxis=rdn_CreditRiskTransferTransactionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |