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Note 13 - Commitments and Contingencies Guarantor Obligations (Details) - Indirect Guarantee of Indebtedness [Member] $ in Millions | Jun. 30, 2018 USD ($) transaction
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Guarantees [Abstract] |
Number of Guaranteed Structured Transactions | transaction | 2
Guarantor Obligations, Maximum Exposure, Undiscounted | $ | $ 92.8

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- Definition Represents the number of guaranteed structured transactions for the entity. + References No definition available. + Details
Name: | rdn_NumberofGuaranteedStructuredTransactions
Namespace Prefix: | rdn_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=82913245&loc=d3e12069-110248 + Details
Name: | us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_GuaranteesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_IndirectGuaranteeOfIndebtednessMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |