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Shareholders' Equity (Details) | Jul. 23, 2015 $ / shares shares
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Class of Warrant or Right [Line Items] |
Common stock, beneficial ownership threshold to be considered an Acquiring Person (as a percent) | 5.00%
Common shares purchasable per Right (in shares) | shares | 0.1
Shareholder rights accompanying each outstanding share of the company's common stock (in number of Rights) | shares | 1
Purchase price (in dollars per share) | $ / shares | $ 45
Purchase price, one-tenth share (in dollars per share) | $ / shares | 4.50
Redemption price (in dollars per Right) | $ / shares | $ 0.001
Rights |
Class of Warrant or Right [Line Items] |
Number of rights per outstanding share of common stock (in shares) | shares | 1
Period after public announcement that a person has become an acquirer (in days) | 10 days
Period after a person announces a tender offer which would make them an acquirer (in days) | 10 days

X
- Definition Class Of Warrant Or Right, Right Per Common Stock Share Outstanding, Number + References No definition available. + Details
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- Definition The number of shares of the company's common stock to which each Right initially entitles a shareholder to purchase. + References No definition available. + Details
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Period Type: | instant

X
- Definition The minimum percentage of beneficial ownership of the company's common stock by any person, by itself or together with its affiliates and associates, required to be considered an Acquiring Person as defined in the Shareholder Rights Agreement. + References No definition available. + Details
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The stated purchase price for which a shareholder is initially entitled to purchase a full share of the company's common stock under the Shareholder Rights Agreement. + References No definition available. + Details
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Namespace Prefix: | mtg_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition The stated purchase price for which a shareholder is initially entitled to purchase a one-tenth share of the company's common stock under the Shareholder Rights Agreement. + References No definition available. + Details
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Namespace Prefix: | mtg_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition The price, per Right, at which each Right is redeemable at any time prior to the time a person becomes an Acquiring Person. + References No definition available. + Details
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Namespace Prefix: | mtg_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition The number of shareholder rights accompanying each outstanding share of the company's common stock under the Shareholder Rights Agreement. + References No definition available. + Details
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Namespace Prefix: | mtg_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Shareholders Rights Agreement, Distribution Date, Period After Announcement Of Tender Offer To Become Acquirer + References No definition available. + Details
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Namespace Prefix: | mtg_
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Balance Type: | na
Period Type: | duration

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- Definition Shareholders Rights Agreement, Distribution Date, Period After Person Becomes Acquiring Person + References No definition available. + Details
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X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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- Details
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Period Type: |