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Note 8 - Reinsurance - Collateral (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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SEC Schedule, 12-17, Insurance Companies, Reinsurance [Abstract] |  |
Funds Held Under Reinsurance Agreements, Off-Balance Sheet, Asset | $ 228.6 | $ 203.2

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- Definition Funds Held Under Reinsurance Agreements, Off-Balance Sheet, Asset + References No definition available. + Details
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Namespace Prefix: | rdn_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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