Statutory Information (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) jurisdiction	Dec. 31, 2016 USD ($)	Dec. 31, 2015 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 (loss)	$ 310,776,000	$ 106,326,000	$ (72,767,000)
Statutory policyholders' surplus	1,622,115,000	1,506,475,000	1,608,214,000
Contingency reserve	$ 1,896,701,000	1,360,088,000	826,706,000
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	10.5 to 1
Risk-to-capital ratio on combined insurance operations	10.5
Percentage of statutory policyholders surplus used to determine maximum allowable dividends	10.00%
Maximum
Statutory capital requirements [Abstract]
Risk to capital ratio	25
Other Insurance Subsidiaries
Related Party Transaction [Line Items]
Dividends paid to the parent company			6,500,000
Distributions from other insurance subsidiaries to the parent company	$ 0	52,001,000	38,500,000
Mortgage Guaranty Insurance Corporation
Related Party Transaction [Line Items]
Loss ratio	5.00%
Additions to the surplus of MGIC from parent company funds	$ 0	36,025,000	0
Dividends paid to the parent company	$ 140,000,000	$ 64,000,000	$ 0
Statutory capital requirements [Abstract]
Risk to capital ratio	9.5
Risk to capital ratio at end of period	9.5 to 1
Amount of policyholders position above or below required MPP	$ 2,100,000,000
Amount of required MPP	$ 1,200,000,000
Adjusted statutory net income measurement period	3 years
Adjusted statutory net income dividend payment measurement period	2 years
Adjustment to statutory net income	$ 473,000,000