Nature of Business (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Nature of Business [Abstract]
Direct domestic primary insurance in force	$ 162,100,000,000
Direct domestic primary risk in force	41,700,000,000
Direct pool risk in force	1,300,000,000
Direct pool risk in force on pool policies with aggregate loss limits	400,000,000
Direct pool risk in force on pool policies with no aggregate loss limits	900,000,000
Risk in force in Australia	680,000,000
Coverage percentage on insurance in force in Australia (in hundredths)	100.00%
Number of jurisdictions with risk-to-capital requirements	16
Maximum permitted risk-to-capital ratio commonly applied	25 to 1
Related Party Transaction [Line Items]
Percentage of new insurance written in jurisdictions with risk-to-capital requirements (in hundredths)	50.00%	50.00%
Amount of policyholders position above or below required MPP	640,000,000
Amount of required MPP	1,200,000,000
Risk-to-capital ratio of combined insurance operations, including reinsurance affiliates, at end of period	47.8 to 1
Percentage of surplus as regards policyholders (in hundredths)	10.00%
Statutory deferred tax assets admitted	63,000,000
Number of active waivers	7
Statutory capital of subsidiary	748,592,000	1,657,349,000	1,692,392,000
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
Mortgage Guaranty Insurance Corporation [Member]
Related Party Transaction [Line Items]
Risk to capital ratio at end of period	44.7 to 1
Amount of policyholders position above or below required MPP	640,000,000
Amount of required MPP	1,200,000,000
Minimum liquid assets required	1,000,000,000
Liquid assets	4,800,000,000
MGIC Indemnity Corporation [Member]
Related Party Transaction [Line Items]
Risk to capital ratio at end of period	1.2 to 1
Statutory capital of subsidiary	448,000,000
New insurance written by MIC	2,400,000,000
Percentage of new insurance that can be written if parent unable to write new business (in hundredths)	100.00%
Number of years for which new insurance can be written by subsidiary	5 years
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]
Percentage of new business that is from jurisdictions in which subsidiary is currently writing business	19.00%	19.00%
Period for honoring claim obligations	5 years
Contribution to subsidiary	$ 100,000,000