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Statutory Information (Details)
 12 Months Ended
Dec. 31, 2018
USD ($)
jurisdiction
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Statutory Capital [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%    
Related Party Transaction [Line Items]      
Statutory net income $ 375,484,000 $ 310,776,000 $ 106,326,000
Statutory policyholders' surplus 1,683,058,000 1,622,115,000 1,506,475,000
Contingency reserve $ 2,442,996,000 1,896,701,000 1,360,088,000
Statutory capital requirements [Abstract]      
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 9.8 to 1    
Risk-to-capital ratio on combined insurance operations 9.8    
Maximum      
Statutory capital requirements [Abstract]      
Risk to capital ratio 25    
Insurance Subsidiaries      
Related Party Transaction [Line Items]      
Distributions from other insurance subsidiaries to the parent company $ 0 0 52,001,000
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    
Mortgage Guaranty Insurance Corporation      
Related Party Transaction [Line Items]      
Loss ratio 4.00%    
Statutory net income $ 325,000,000    
Statutory policyholders' surplus 1,682,000,000    
Additions to the surplus of MGIC from parent company funds 0 0 36,025,000
Dividends paid to the parent company $ 220,000,000 $ 140,000,000 $ 64,000,000
Statutory capital requirements [Abstract]      
Risk to capital ratio 9.0    
Risk to capital ratio at end of period 9.0 to 1    
Amount of policyholders position above or below required MPP $ 2,600,000,000    
Amount of required MPP 1,300,000,000    
Reduction in statutory net income due to increase in contingency reserve $ 484,000,000