Note 17 - Share-Based Compensation and Other Benefit Programs - Employee Stock Purchase Plan (Details) - Amended and Restated Radian Group Inc. Employee Stock Purchase Plan - Employee Stock Purchase Plan - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 01, 2021
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected life (months)	6 months
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date	15.00%
Shares sold to employees under ESPP Plans	100,022	107,009	103,668
Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Employee Stock Purchase Plan (ESPP), Number of Shares Available for Issuance				1,776,532