SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - SUPPLEMENTARY NOTES (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Insurance Subsidiaries
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Percentage of statutory policyholders surplus used to determine maximum allowable dividends			10.00%
Adjusted statutory net income measurement period			3 years
Adjusted statutory net income dividend payment measurement period			2 years
Proceeds from contribution from parent			$ 0	$ 0	$ 0
Convertible Junior Subordinated Debentures, at 9% per annum, Due 2063
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Stated interest rate (in hundredths)			9.00%
Mortgage Guaranty Insurance Corporation
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Dividends paid to the parent company			$ 280,000,000	$ 220,000,000	$ 140,000,000
Mortgage Guaranty Insurance Corporation | Convertible Junior Subordinated Debentures, at 9% per annum, Due 2063
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Investment in holding company debt			$ 132,700,000
Forecast | Mortgage Guaranty Insurance Corporation
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Dividends paid to the parent company	$ 70,000,000	$ 280,000,000
Special dividend paid	$ 320,000,000