Statutory Information (Details) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) jurisdiction	Sep. 30, 2018 USD ($) jurisdiction	Dec. 31, 2017 USD ($)
Statutory capital requirements [Abstract]
Number of jurisdictions with risk-to-capital requirements (in jurisdictions) | jurisdiction	16	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		9.8 to 1
Risk-to-capital ratio for combined insurance operations	9.8	9.8
Percentage of statutory policyholders surplus used to determine maximum allowable dividends (as a percent)	10.00%	10.00%
Mortgage Guaranty Insurance Corporation
Statutory capital requirements [Abstract]
Maximum risk-to-capital ratio		25
Risk to capital ratio at end of period		9.0 to 1
Risk-to-capital ratio	9.0	9.0
Amount of policyholders position above or below required MPP	$ 2,500	$ 2,500
Amount of required MPP	1,300	$ 1,300
Dividends paid to the parent company	$ 60
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
Statutory net income			$ 272