We cannot pay deferred interest other than from the net proceeds of Alternative Payment Mechanism sales, except at the final maturity of the debentures or at the tenth anniversary of the start of the interest deferral.

Name:	mtg_AnniversaryPaymentReleaseOfStartOfInterestDeferralToAlternativePaymentMechanismInLieuOfFinalMaturityOfDebentures
Namespace Prefix:	mtg
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	instant

- Definition

The period after a principal or interest payment is due within which a payment must be made to avoid a default pursuant to the restrictive covenants under the note agreement(s).

+ Details

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

- Definition

The amount of cash paid for interest that was previously deferred pursuant to the optional deferral provision under the note agreement(s).

+ Details

Name:	mtg_DeferredInterestPaymentMade
Namespace Prefix:	mtg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The gain on repurchase of principal outstanding on a debt instrument.

+ Details

Name:	mtg_GainOnRepurchaseDebtInstrument
Namespace Prefix:	mtg
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Date when the debt instrument is scheduled to be fully repaid, which may be presented in a variety of ways (year, month and year, day, month and year, quarter, etc.).

+ Details

Name:	mtg_MaturityDate
Namespace Prefix:	mtg
Data Type:	invest:dateStringItemType
Balance Type:	na
Period Type:	instant

- Definition

The maximum number of shares of common stock, including shares underlying qualifying warrants, that the entity may be required to issue pursuant to the Alternative Payment Mechanism under the note agreement(s).

+ Details

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

- Definition

The maximum number of trading days within a specified time period that the closing sale price of the entity's common stock exceeds the prevailing conversion price of the debentures by a specified percentage.

+ Details

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

- Definition

The maximum ratio of total net proceeds of all issuances of qualifying securities to the aggregate principal amount of the debentures. Pursuant to the Alternative Payment Mechanism under the note agreement(s), the entity may not issue qualifying preferred stock if the total net proceeds exceed this ratio.

+ Details

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

- Definition

The minimum number of consecutive periods for which interest payments may be deferred pursuant to an optional deferral provision under the note agreement(s).

+ Details

Name:	mtg_MinimumNumberOfConsecutivePeriodsForWhichInterestPaymentsMayBeDeferred
Namespace Prefix:	mtg
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

The minimum number of trading days within a specified time period that the closing sale price of the entity's common stock exceeds the prevailing conversion price of the debentures by a specified percentage.

+ Details

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

- Definition

The minimum percentage of notes held by holders, in the aggregate, that would allow such holders the right to accelerate the maturity of a given series of notes pursuant to the restrictive covenants under the note agreement(s).

+ Details

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

- Definition

The minimum percentage of consolidated shareholders' equity that a subsidiary's shareholders' equity must represent to be deemed a designated subsidiary as defined by the note agreements.

+ Details

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

- Definition

The percentage calculated as the average closing price of the entity's common stock times the number of the entity's outstanding shares of common stock. The average price is determined over a specified period ending before the issuance of the common stock or warrants being sold, and the number of outstanding shares is determined as of the date of the entity's most recent publicly released financial statements.

+ Details

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

- Definition

The period before an interest payment is due prior to which the entity is required to give notice of its intent to defer payment pursuant to the optional deferral provision under the note agreement(s).

+ Details

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

- Definition

The ratio of the closing sale price of the entity's common stock over the prevailing conversion price of the debentures as of the redemption date.

+ Details

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

- Definition

The maximum period, in business days, after payment of interest on debentures that was not deferred, in which the entity must begin reasonable commercial efforts to sell qualifying securities to unaffiliated persons pursuant to the optional deferral provision under the note agreement(s).

+ Details

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

- Definition

The minimum period, in business days, after payment of interest on debentures that was not deferred, in which the entity must begin reasonable commercial efforts to sell qualifying securities to unaffiliated persons pursuant to the optional deferral provision under the note agreement(s).

+ Details

Name:	mtg_PeriodInWhichReasonableCommercialEffortsMustBeginMinimumInBusinessDays
Namespace Prefix:	mtg
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	instant

- Definition

The period immediately prior to an election to convert debentures during which the average price of shares traded is determined for the purpose of calculating the conversion rate.

+ Details

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

- Definition

The principal amount of notes used in the determination of the initial conversion rate, which is subject to adjustment.

+ Details

Name:	mtg_PrincipalAmountOfNotesUsedInDeterminingConversionRate
Namespace Prefix:	mtg
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The percentage of the principal amount of the debentures being redeemed which would equal the redemption price under certain conditions as specified in the note agreement(s).

+ Details

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

- Definition

The minimum aggregate amount of other debt that, if maturity were accelerated, would result in a default pursuant to the restrictive covenants under the note agreement(s).

+ Details

Name:	mtg_RestrictionOnAccelerationOfOtherDebt
Namespace Prefix:	mtg
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The minimum aggregate amount of payments due with respect to a final judgment rendered against the entity or any of its subsidiaries and which is not discharged or stayed within certain time limits that would result in a default pursuant to the restrictive covenants under the note agreement(s).

+ Details

Name:	mtg_RestrictionOnFinalJudgments
Namespace Prefix:	mtg
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Including the current and noncurrent portions, the carrying value of convertible subordinated debt, as of the balance sheet date, initially scheduled to be repaid after one year or beyond the normal operating cycle if longer. This form of debt can be exchanged for a specified amount of another security, typically the entity's common stock, at the option of the issuer or the holder, and places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 5

-Section 02

-Subsection 20, 22

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX210.5-02.22))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_ConvertibleSubordinatedDebt
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The price per share of the conversion feature embedded in the debt instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 20

-Section 50

-Paragraph 5

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6928298&loc=SL6031898-161870

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number APB14-1

-Paragraph 32

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Name:	us-gaap_DebtInstrumentConvertibleConversionPrice1
Namespace Prefix:	us-gaap
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

The ratio applied to the debt for purposes of determining the number of shares of the equity security into which the debt will be converted.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21521-112644

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(5))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Conversion Rate

-URI http://asc.fasb.org/extlink&oid=6509012

+ Details

Name:	us-gaap_DebtInstrumentConvertibleConversionRatio
Namespace Prefix:	us-gaap
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

The effective interest rate on the liability component of convertible debt instrument which may be settled in cash upon conversion, including partial cash settlement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 20

-Section 50

-Paragraph 6

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6928298&loc=SL6036836-161870

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number APB14-1

-Paragraph 33

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Name:	us-gaap_DebtInstrumentConvertibleEffectiveInterestRate
Namespace Prefix:	us-gaap
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Decrease for amounts repaid on the debt instrument for the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 4

-Section 08

-Paragraph f

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(f))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

+ Details

Name:	us-gaap_DebtInstrumentDecreaseRepayments
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 21

-Paragraph 16, 20

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13476-108611

+ Details

Name:	us-gaap_DebtInstrumentFairValue
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Interest rate stated in the contractual debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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.

+ Details

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

- Definition

The amount of cash paid for interest during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 29

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 25

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3536-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4297-108586

+ Details

Name:	us-gaap_InterestPaid
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Debt (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified			9 Months Ended			9 Months Ended		9 Months Ended		9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2011 Senior Notes [Member]	Sep. 30, 2010 Senior Notes [Member]	Dec. 31, 2010 Senior Notes [Member]	Sep. 30, 2011 Senior Notes [Member] Senior Notes Due 2011 [Member]	Dec. 31, 2010 Senior Notes [Member] Senior Notes Due 2011 [Member]	Sep. 30, 2011 Senior Notes [Member] Senior Notes Due 2015 [Member]	Dec. 31, 2010 Senior Notes [Member] Senior Notes Due 2015 [Member]	Sep. 30, 2011 Convertible Senior Notes Due 2017 [Member]	Dec. 31, 2010 Convertible Senior Notes Due 2017 [Member]	Sep. 30, 2011 Convertible Junior Subordinated Debentures Due 2063 [Member]	Dec. 31, 2010 Convertible Junior Subordinated Debentures Due 2063 [Member]
Debt Instrument [Line Items]
Outstanding principal amount						$ 0	$ 77,400,000	$ 245,000,000	$ 300,000,000	$ 345,000,000	$ 345,000,000	$ 389,500,000	$ 389,500,000
Stated interest rate (in hundredths)						5.625%	5.625%	5.375%	5.375%	5.00%	5.00%	9.00%	9.00%
Maturity date						September 2011	September 2011	November 2015	November 2015	May 1, 2017	May 1, 2017	2063	2063
Outstanding principal amount repaid/repurchased						77,400,000		55,000,000
Gain on repurchase of debt								3,200,000
Minimum percentage of consolidated shareholders' equity that determines designated subsidiary status (in hundredths)			15.00%
Default period (in days)			30							30
Restriction on acceleration of other debt			40,000,000							40,000,000
Minimum percentage of aggregate notes held to grant acceleration rights to note holders (in hundredths)			25.00%							25.00%
Fair value of the amount outstanding			177,600,000		355,600,000					193,200,000	400,500,000	170,700,000	432,400,000
Interest payments made			12,500,000	12,500,000						8,600,000	0	17,500,000	0
Restriction on final judgments										40,000,000
Conversion rate (in shares per $1,000 note)										74.4186		74.0741
Principal amount of notes used in determining conversion rate										1,000		1,000
Initial conversion price (in dollars per share)										$ 13.44		$ 13.50
Effective interest rate (in hundredths)												19.00%	19.00%
Amortized value of notes recorded as a liability on the consolidated balance sheet	336,694,000	315,626,000										336,700,000	315,600,000
Minimum number of consecutive periods for which interest payments may be deferred (in years)												1
Deferred interest payments made												$ 57,500,000
Notice period requirement for deferral of interest payment (in days)												15
Period in which reasonable commercial efforts must begin, maximum (in business days)												1
Period in which reasonable commercial efforts must begin, minimum (in business days)												fifth anniversary of the scheduled interest payment
Anniversary payment release of the start of the interest deferral to the Alternative Payment Mechanism in lieu of the final maturity of the debentures												tenth
Net proceeds cap (in hundredths)												2.00%
Maximum number of shares of common stock issuable under the Alternative Payment Mechanism												10
Maximum percentage of aggregate principal amount of the debentures (in hundredths)												25.00%
Redemption price, percentage (in hundredths)												100.00%
Percentage of conversion price (in hundredths)												130.00%
Minimum number of trading days (in days)												20
Maximum number of trading days (in days)												30
Period preceding election to convert (in days)												5