Note 14 - Capital Stock - Share-based Compensation (Details)	9 Months Ended
Sep. 30, 2020 anniversary $ / shares shares	Dec. 31, 2019 $ / shares shares
Performance Based RSUs Equity Settled
Number of Shares, Unvested \| shares	2,218,334	2,448,347
Number of Shares, Granted \| shares	1,175,062
Number of Shares, Performance adjustment \| shares	(788,256)
Number of Shares, Vested in Period \| shares	(591,118)
Number of Shares, Forfeited \| shares	(25,701)
Weighted Average Grant Date Fair Value, Unvested \| $ / shares	$ 14.68	$ 17.03
Weighted Average Grant Date Fair Value, Granted \| $ / shares	12.02
Weighted Average Grant Date Fair Value, Performance Adjustment \| $ / shares	0
Weighted Average Grant Date Fair Value, Vested \| $ / shares	17.59
Weighted Average Grant Date Fair Value, Forfeited \| $ / shares	$ 17.90
Performance Based RSUs Equity Settled \| 2020 Award Year
Performance Period, Performance-based RSUs	3 years
Performance Based RSUs Equity Settled \| 2020 Award Year \| Maximum
Payout Percentage of Target Award	200.00%
Performance Based RSUs Equity Settled \| 2020 Award Year \| Minimum
Payout Percentage of Target Award	0.00%
Time Vested RSUs Equity Settled
Number of Shares, Unvested \| shares	952,953	697,576
Number of Shares, Granted \| shares	709,598
Number of Shares, Performance adjustment \| shares	0
Number of Shares, Vested in Period \| shares	(446,966)
Number of Shares, Forfeited \| shares	(7,255)
Weighted Average Grant Date Fair Value, Unvested \| $ / shares	$ 16.45	$ 19.72
Weighted Average Grant Date Fair Value, Granted \| $ / shares	13.72
Weighted Average Grant Date Fair Value, Performance Adjustment \| $ / shares	0
Weighted Average Grant Date Fair Value, Vested \| $ / shares	17.17
Weighted Average Grant Date Fair Value, Forfeited \| $ / shares	$ 19.12
Time Vested RSUs Equity Settled \| 2020 Award Year
Number of Anniversaries of the Grant Date \| anniversary	3
Time Vested RSUs Equity Settled \| 2020 Award Year \| Minimum
Cliff-Vesting Period	1 year

Number of Anniversaries of the Grant Date

No definition available.

Name:	rdn_NumberOfAnniversariesOfTheGrantDate
Namespace Prefix:	rdn_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Performance Adjustment to Shares, Weighted Average Grant Date Fair Value

+ References

No definition available.

+ Details

Name:	rdn_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceAdjustmentToSharesWeightedAverageGrantDateFairValue
Namespace Prefix:	rdn_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Share-based Compensation Maximum Payout Percentage of Target Award

+ References

No definition available.

+ Details

Name:	rdn_ShareBasedCompensationMaximumPayoutPercentageOfTargetAward
Namespace Prefix:	rdn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(3)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(2)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(2)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares under non-option equity instrument agreements that were cancelled as a result of occurrence of a terminating event.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsForfeitures
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Net number of non-option equity instruments granted to participants.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Other increase (decrease) in number of shares reserved for issuance under non-option equity instrument agreements that is not separately disclosed.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOther
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=rdn_PerformanceBasedRSUsEquitySettledMemberMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=rdn_A2020AwardYearMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=rdn_TimeVestedRSUsEquitySettledMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: