SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - SUPPLEMENTARY NOTES (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Insurance Subsidiaries
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
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]
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
Dividends paid to the parent company	$ 390,000,000	280,000,000	$ 220,000,000
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