Basis of Presentation (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013 Jurisdiction	Dec. 31, 2012
Basis of Presentation [Abstract]
Number of jurisdictions with risk-to-capital requirements	16
Related Party Transaction [Line Items]
Contribution To Subsidiary	$ 800,000,000
Direct domestic primary risk in force	800,000,000
Amount of policyholders position above or below required MPP	168,000,000
Amount of required MPP	1,200,000,000
Risk-to-capital ratio of combined insurance operations, including reinsurance affiliates, at end of period	23.1 to 1
Percentage of new insurance written in jurisdictions with risk-to-capital requirements (in hundredths)	50.00%	50.00%
Maximum risk-to-capital requirement of a subsidiary under a GSE agreement	18:1
Number of days in which review needs to be completed from triggering event	60 days
Dividend paid period	30 days
Percentage of surplus as regards policyholders (in hundredths)	10.00%
Statutory deferred tax assets admitted	136,000,000	63,000,000
Mortgage Guaranty Insurance Corporation [Member]
Related Party Transaction [Line Items]
Maximum permitted risk-to-capital ratio commonly applied	25 to 1
Risk to capital ratio at end of period	20.4 to 1
MGIC Indemnity Corporation [Member]
Related Party Transaction [Line Items]
Risk to capital ratio at end of period	1.8 to 1
Statutory capital of subsidiary	$ 450,000,000
Number of days for which mortgage insurance can be written in jurisdictions that do not have capital requirements if parent is unable to do so	60 days
MGIC Investment Corporation [Member]
Related Party Transaction [Line Items]
Period for honoring claim obligations	5 years