Nature of business (Details) (USD $)	12 Months Ended			1 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jan. 31, 2012 Mortgage Guaranty Insurance Corporation [Member]	Dec. 31, 2011 Mortgage Guaranty Insurance Corporation [Member]	Dec. 31, 2009 Mortgage Guaranty Insurance Corporation [Member]	Jan. 31, 2012 MGIC Indemnity Corporation [Member]	Dec. 31, 2009 MGIC Indemnity Corporation [Member]	Dec. 31, 2011 MGIC Indemnity Corporation [Member]	Dec. 31, 2011 MGIC Investment Corporation [Member]
Nature of business [Abstract]
Direct domestic primary insurance in force	$ 172,900,000,000
Direct domestic primary risk in force	44,500,000,000
Direct pool risk in force	1,900,000,000
Direct pool risk in force on pool policies with aggregate loss limits	700,000,000
Direct pool risk in force on pool policies with no aggregate loss limits	1,200,000,000
Risk in force in Australia	900,000,000
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]
Contribution to subsidiary				200,000,000	200,000,000	200,000,000	200,000,000	200,000,000		200,000,000
Holding company cash and investments										487,000,000
MGIC's risk-to-capital ratio at end of period					20.3 to 1
Excess of policyholders position over required regulatory minimum					185,000,000
Risk-to-capital ratio of combined insurance operations, including reinsurance affiliates, at end of period	22.2 to 1
Statutory deferred tax assets					142,000,000
Minimum amount of liquid assets to be maintained under the New Order					1,000,000,000
Liquid assets					6,400,000,000
Percentage of new insurance written in jurisdictions with risk-to-capital requirements (in hundredths)					50.00%
Statutory capital of subsidiary	1,657,349,000	1,692,392,000	1,442,407,000						234,000,000
Minimum number of jurisdictions that could prevent the entity from continuing to write new insurance		1
Maximum risk-to-capital requirement of a subsidiary under a GSE agreement									20:1
Claim payments to be paid by competitor mortgage insurance companies (in hundredths)	50.00%
Mitigation of paid losses by rescission of policies	600,000,000	1,200,000,000	1,200,000,000
Percentage of claims received in a quarter resolved by rescission, lower range limit (in hundredths)	17.00%
Percentage of claims received in a quarter resolved by rescission, upper range limit (in hundredths)	20.00%		28.00%
Estimated mitigation of incurred losses by impact of rescissions on loss reserving methodology		$ 200,000,000	$ 2,500,000,000
Number of loans in primary delinquency inventory	175,639
Period during which the insured may initiate legal proceedings (in years)	3Y