Fair Value Measurements, Unobservable Input Reconciliation (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Total realized/unrealized gains (losses) [Abstract]
State premium tax credit investments, average maturity	4 years
Annual average discount rate (in hundredths)	7.20%mtg_FairValueAnnualAverageDiscountRate
Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 1,846	$ 2,423
Total realized/unrealized gains (losses) [Abstract]
Included in earnings and reported as losses incurred, net	0	0
Purchases	7	30
Sales	(62)	(75)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance at end of period	1,791	2,378
Amount of total losses included in earnings attributable to the change in unrealized losses on assets still held	0	0
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	321	321
Total realized/unrealized gains (losses) [Abstract]
Included in earnings and reported as losses incurred, net	0	0
Purchases	0	0
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance at end of period	321	321
Amount of total losses included in earnings attributable to the change in unrealized losses on assets still held	0	0
Total Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	2,167	2,744
Total realized/unrealized gains (losses) [Abstract]
Included in earnings and reported as losses incurred, net	0	0
Purchases	7	30
Sales	(62)	(75)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance at end of period	2,112	2,699
Amount of total losses included in earnings attributable to the change in unrealized losses on assets still held	0	0
Real Estate Acquired [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	12,658	13,280
Total realized/unrealized gains (losses) [Abstract]
Included in earnings and reported as losses incurred, net	(503)	(1,160)
Purchases	10,797	8,010
Sales	(12,055)	(8,993)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance at end of period	10,897	11,137
Amount of total losses included in earnings attributable to the change in unrealized losses on assets still held	$ 0	$ 0

This element represents annual average discount rate used for fair value of state premium tax credit investments.

No definition available.

Name:	mtg_FairValueAnnualAverageDiscountRate
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

This element represents total losses for the period (realized and unrealized), arising from assets measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in earnings or resulted in a change in net asset value and are attributable to the change in unrealized losses on assets still held at the end of the period.

+ References

No definition available.

+ Details

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

- Definition

This element represents state premium tax credit investments, average maturity.

+ References

No definition available.

+ Details

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

- Details

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- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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