Statutory Information (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) jurisdiction	Dec. 31, 2017 USD ($)
Statutory capital requirements [Abstract]
Number of jurisdictions with risk-to-capital requirements (in jurisdictions) | jurisdiction	16
Maximum permitted risk-to-capital ratio commonly applied	25 to 1
Risk to capital ratio of combined insurance operations, including reinsurance affiliates, at end of period	10.3 to 1
Risk-to-capital ratio for combined insurance operations	10.3
Percentage of statutory policyholders surplus used to determine maximum allowable dividends (as a percent)	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
Amount of policyholders position above or below required MPP	$ 2,200
Amount of required MPP	1,200
Cash dividends paid	$ 50
Adjusted statutory net income measurement period (in years)	3 years
Adjusted statutory net income dividend payment measurement period (in years)	2 years
Reduction in statutory net income due to increase in contingency reserve		$ 473