Note 1 - Description of Business (Details) $ in Millions	9 Months Ended
Sep. 30, 2021 USD ($) segment	Dec. 31, 2020 USD ($)
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	$ 246,100.0
Direct Primary Mortgage Insurance RIF	59,400.0	60,700.0
Credit Risk Transfer Transactions \| Mortgage segment
Mortgage Insurance [Abstract]
Direct Primary Mortgage Insurance RIF	$ 429.3	$ 392.0

Business Overview [Abstract]

No definition available.

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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

- 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:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- 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:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- 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

- 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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=123359005&loc=d3e8672-108599

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- 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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	rdn_RiskinForceAxis=rdn_CreditRiskTransferTransactionsMember
Namespace Prefix:
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Period Type: