Statutory Information (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) jurisdiction	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Statutory capital requirements [Abstract]
Percentage of premiums earned required to be maintained as contingency loss reserves (in hundredths)			50.00%
Period that contingency loss reserves must be held (in years)			10 years
Percentage of net premiums earned that incurred losses must exceed to enable early withdrawals from contingency loss reserves (in hundredths)			35.00%
Statutory net income			$ 305,857,000	$ 375,484,000	$ 310,776,000
Statutory policyholders' surplus			1,619,069,000	1,683,058,000	1,622,115,000
Contingency reserve			$ 3,021,055,000	2,442,996,000	1,896,701,000
Number of jurisdictions with risk-to-capital requirements | jurisdiction			16
Maximum permitted risk-to-capital ratio commonly applied			25 to 1
Risk-to-capital ratio on a combined basis at end of period			9.6 to 1
Risk-to-capital ratio on combined insurance operations			9.6
Maximum
Statutory capital requirements [Abstract]
Risk to capital ratio			25
Mortgage Guaranty Insurance Corporation
Statutory capital requirements [Abstract]
Loss ratio			12.00%
Statutory net income			$ 273,000,000
Statutory policyholders' surplus			1,619,000,000
Dividends paid to the parent company			$ 280,000,000	$ 220,000,000	$ 140,000,000
Risk to capital ratio			9.7
Risk to capital ratio at end of period			9.7 to 1
Amount of policyholders position above or below required MPP			$ 3,000,000,000.0
Amount of required MPP			1,700,000,000
Reduction in statutory net income due to increase in contingency reserve			$ 556,000,000
Forecast | Mortgage Guaranty Insurance Corporation
Statutory capital requirements [Abstract]
Dividends paid to the parent company	$ 70,000,000	$ 280,000,000
Special dividend paid	$ 320,000,000
Insurance Subsidiaries
Statutory capital requirements [Abstract]
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