Basis of presentation (Details) (USD $)	3 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Jun. 30, 2009	Sep. 30, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jun. 30, 2012	Sep. 30, 2011	Jun. 30, 2011
Basis of presentation [Abstract]
Number of jurisdictions with risk-to-capital requirements	16		16
Percentage of new insurance written in jurisdictions with risk-to-capital requirements (in hundredths)			50.00%	50.00%
Maximum permitted risk-to-capital ratio commonly applied			25 to 1
Risk to capital ratio at end of period			31.5 to 1
Risk to capital ratio of MIC at end of period			0.3 to 1
Amount of policyholders position above or below required MPP	$ 344,000,000		$ 344,000,000
Amount of required MPP	1,300,000,000		1,300,000,000
Risk-to-capital ratio of combined insurance operations, including reinsurance affiliates, at end of period			34.1 to 1
Statutory Deferred Tax Assets Admitted	90,000,000		90,000,000
Minimum Liquid Assets required	1,000,000,000		1,000,000,000
Liquid Assets	5,100,000,000		5,100,000,000
Waiver received, number of jurisdiction			8
No current waivers in effect, number of jurisdiction			4
Number of jurisdiction with pending response			3
Percentage of surplus as regards policyholders	10.00%		10.00%
Contribution to subsidiary	200,000,000		200,000,000
Statutory capital of subsidiary	443,000,000		443,000,000
New Insurance Written By MIC	587,000,000
Maximum risk-to-capital ratio allowed for MIC			20:1
Future Contribution to MGIC	100,000,000		100,000,000
Settlement amount			267,500,000
Settlement amount due upon effectiveness of settlement			100,000,000
Settlement amount to be paid in monthly installments			167,500,000
Number of monthly installment payments on insurance settlement			48
Contribution from holding company			100,000,000
Number of jurisdiction that could impact the writing of new insurance	1		1
Number of competitors subject to partial claim payment plans	2		2
Mitigation of paid losses by rescission of policies			200,000,000	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)			8.00%			8.00%
Percentage of claims received in a quarter resolved by rescission, upper range limit (in hundredths)		28.00%	13.00%			13.00%
Number of jurisdictions MIC approved by Freddie Mac	15		15
Number of rescindable loans affected by Company's decision to suspend rescissions	1,700		1,700
Total potential claim payments of rescindable loans affected by Company's decision to suspend rescissions	125,000,000		125,000,000
Number of rescindable loans affected by Company's decision to suspend rescissions for customers other than Countrywide	350		350
Total potential claim payments of rescindable loans affected by Company's decision to suspend rescissions for customers other than Countrywide	23,000,000		23,000,000
Estimated mitigation of incurred losses by impact of rescissions on loss reserving methodology					$ 200,000,000	$ 2,500,000,000
Primary Default Inventory	148,885		148,885	175,639	214,724		153,990	180,894	184,452
Period during which the insured may initiate legal proceedings			3 years
Number of settlement agreements approved by GSEs	1		1