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Note 1 - Condensed Consolidated Financial Statements - Business Overview, Recent Developments and Significant Accounting Policies Business Overview (Details) $ in Billions	3 Months Ended
	Mar. 31, 2019 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%
Direct Primary Mortgage Insurance RIF | $	$ 57.4

X

- Definition

Business Overview [Abstract]

+ References

No definition available.

+ Details

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Namespace Prefix:	rdn_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Mortgage Insurance [Abstract]

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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

X

- 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.

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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

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No definition available.

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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.

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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:
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Period Type: