Conversion Rate Adjustment, Threshold

No definition available.

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- 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.

+ References

No definition available.

+ Details

Name:	mtg_PeriodPrecedingElectionToConvert
Namespace Prefix:	mtg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	mtg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reference dividend amount to determine conversion rate adjustment calculation.

+ References

No definition available.

+ Details

Name:	mtg_ReferenceDividendAmount
Namespace Prefix:	mtg_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031898-161870

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio applied to the conversion of debt instrument into equity with equity shares divided by debt principal amount.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
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+ Details

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Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 210
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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.

+ References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
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Period Type:	duration

- Definition

Amount of cash paid for interest, including, but not limited to, capitalized interest and payment to settle zero-coupon bond attributable to accreted interest of debt discount and debt instrument with insignificant coupon interest rate in relation to effective interest rate of borrowing attributable to accreted interest of debt discount; classified as operating and investing activities.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4297-108586

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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-Paragraph 1
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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- Details

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Debt - Narrative (Details)	9 Months Ended
Debt - Narrative (Details)	Sep. 30, 2019 USD ($) $ / shares	Sep. 30, 2018 USD ($)	May 31, 2019 USD ($)
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity			$ 175,000,000
Total interest payments	$ 38,500,000	$ 38,800,000
9% Convertible Junior Subordinated Debentures
Debt Instrument [Line Items]
Conversion rate (in shares per $1,000 note)	0.0740741
Principal amount of notes used in determining conversion price per share	$ 1,000
Convertible debt, conversion price (in dollars per share) \| $ / shares	$ 13.50
Reference dividend amount (per share) \| $ / shares	$ 0.025
Conversion rate adjustment, threshold	1.00%
Period preceding election to convert (in days)	5 days
Stated interest rate	9.00%
Senior Notes \| 5.75% Senior Notes Due 2023
Debt Instrument [Line Items]
Stated interest rate	5.75%