Shareholders' equity (Details) (USD $)	9 Months Ended
Sep. 30, 2011
Shareholders' equity [Abstract]
Number of rights accompanying each outstanding share of Common Stock	1
Distribution date after a public announcement (in days)	10D
An Acquiring Person's beneficial ownership (in hundredths)	5.00%
Share of common stock each right will initially entitle shareholders to buy (in shares)	0.5
Purchase Price per full share (in dollars per share)	$ 25
Equivalent price for each one-half share (in dollars per share)	$ 12.50
Rights expiration date	2012-08-17
Rights redeemable price ( per unit)	$ 0.001

The percentage or more of outstanding common stock that a beneficial owner should have in order to become an acquiring person.

Name:	mtg_AnAcquiringPersonsBeneficialOwnership
Namespace Prefix:	mtg
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

With respect to rights outstanding as of the balance sheet date, this item provides disclosure of the date from which rights are exercisable.

Name:	mtg_DistributionDateAfterPublicAnnouncement
Namespace Prefix:	mtg
Data Type:	invest:dateStringItemType
Balance Type:	na
Period Type:	duration

The price at which each right will buy one-half of one share of common stock.

Name:	mtg_EquivalentPriceForEachOneHalfShare
Namespace Prefix:	mtg
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

The number of rights that accompany each outstanding share of the entity's common stock.

Name:	mtg_NumberOfRightsAccompanyingEachOutstandingShareOfCommonStock
Namespace Prefix:	mtg
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

The purchase price for each full share of the entity's common stock acquired with rights.

Name:	mtg_PriceForEachFullShareOfCommonStockPurchasedWithRights
Namespace Prefix:	mtg
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

The redemption price of each right.

Name:	mtg_RedemptionPriceOfRight
Namespace Prefix:	mtg
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	instant

The expiry date of rights issued.

The number of shares of common stock that each right will initially entitle shareholders to buy.

Name:	mtg_ShareOfCommonStockEachRightWillInitiallyEntitleShareholdersToBuy
Namespace Prefix:	mtg
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

Name:	us-gaap_StockholdersEquityNoteAbstract
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration