Statutory Information - Narrative (Details)	3 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) jurisdiction	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
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%
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 of combined insurance operations at end of period		9.1 to 1
Risk-to-capital ratio on combined insurance operations		9.1
Statutory net income (loss)		$ 65,201,000	$ 305,857,000	$ 375,484,000
Statutory policyholders' surplus		$ 1,339,509,000	1,619,069,000	1,683,058,000
Maximum
Statutory capital requirements [Abstract]
Risk to capital ratio		25
Mortgage Guaranty Insurance Corporation
Statutory capital requirements [Abstract]
Contingency reserve withdrawal		$ 30,400,000
Risk to capital ratio at end of period		9.2 to 1
Risk to capital ratio		9.2
Amount of policyholders position above or below required MPP		$ 3,200,000,000
Amount of required MPP		1,700,000,000
Dividends paid to the parent company		$ 390,000,000	$ 280,000,000	$ 220,000,000
Dividend to parent, distribution of interest in 9% Debentures	$ 139,500,000
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
Reduction in statutory net income due to increase in contingency reserve		$ 559,000,000
Statutory net income (loss)		(643,000,000)
Statutory policyholders' surplus		1,336,000,000
Realized statutory loss on merger of affiliate, eliminated in consolidation		$ 692,000,000