Statutory Information (Details) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2020 USD ($) jurisdiction	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($) jurisdiction
Statutory capital requirements [Abstract]
Number of jurisdictions with risk-to-capital requirements (in jurisdictions) | jurisdiction	16			16
Maximum permitted risk-to-capital ratio commonly applied				25 to 1
Risk to capital ratio of combined insurance operations at end of period				9.4 to 1
Risk-to-capital ratio for combined insurance operations	9.4			9.4
Percentage of statutory policyholders surplus used to determine maximum allowable dividends (as a percent)	10.00%			10.00%
Mortgage Guaranty Insurance Corporation
Statutory capital requirements [Abstract]
Maximum risk-to-capital ratio				25
Risk to capital ratio at end of period				9.4 to 1
Risk-to-capital ratio	9.4			9.4
Amount of policyholders position above or below required MPP	$ 3,100.0			$ 3,100.0
Amount of required MPP	1,700.0			$ 1,700.0
Dividends paid to the parent company	0.0	$ 0.0	$ 390.0
Dividend to parent, distribution of interest in 9% Debentures	$ 139.7
Adjusted statutory net income measurement period (in years)				3 years
Adjusted statutory net income dividend payment measurement period (in years)				2 years