Loss Reserves - Aging of Primary Default Inventory and Number of Delinquent Payments (Details) - loan	3 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Abstract]
Default notices for loans in IADAs	9,294	1,968
Primary Default Inventory In FEMA Individual Assistance Disaster Areas	12,446	7,162
Aging of the Primary Default Inventory [Abstract]
3 months or less	17,119	12,194	13,053
3 months or less (in hundredths)	37.00%	24.00%	21.00%
4 - 11 months	12,050	13,450	15,763
4 - 11 months (in hundredths)	26.00%	27.00%	25.00%
12 months or more	17,387	24,638	33,817
12 months or more (in hundredths)	37.00%	49.00%	54.00%
Total primary default inventory	46,556	50,282	62,633	79,901
Total primary default inventory (in hundredths)	100.00%	100.00%	100.00%
Primary claims received inventory included in ending default inventory	954	1,385	2,769
Primary claims received inventory included in ending default inventory (in hundredths)	2.00%	3.00%	4.00%
Number of Payments Delinquent [Abstract]
3 payments or less	21,678	18,419	20,360
3 payments or less (in hundredths)	46.00%	36.00%	33.00%
4 - 11 payments	12,446	12,892	15,092
4 - 11 payments (in hundredths)	27.00%	26.00%	24.00%
12 payments or more	12,432	18,971	27,181
12 payments or more (in hundredths)	27.00%	38.00%	43.00%
Total primary default inventory	46,556	50,282	62,633	79,901
Total primary default inventory (in hundredths)	100.00%	100.00%	100.00%

No definition available.

Name:	mtg_AgingOfPrimaryDefaultInventoryAbstract
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of loans in the primary default inventory that have been delinquent for twelve payments or more.

+ References

No definition available.

+ Details

Name:	mtg_NumberOfPaymentsDelinquent12PaymentsOrMore
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of loans in the primary default inventory that have been delinquent for twelve payments or more.

+ References

No definition available.

+ Details

Name:	mtg_NumberOfPaymentsDelinquent12PaymentsOrMorePercentage
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of loans in the primary default inventory that have been delinquent for three payments or less.

+ References

No definition available.

+ Details

Name:	mtg_NumberOfPaymentsDelinquent3PaymentsOrLess
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of loans in the primary default inventory that have been delinquent for three payments or less.

+ References

No definition available.

+ Details

Name:	mtg_NumberOfPaymentsDelinquent3PaymentsOrLessPercentage
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of loans in the primary default inventory that have been delinquent for at least four payments but no more than eleven payments.

+ References

No definition available.

+ Details

Name:	mtg_NumberOfPaymentsDelinquent4To11Payments
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of loans in the primary default inventory that have been delinquent for at least four payments but no more than eleven payments.

+ References

No definition available.

+ Details

Name:	mtg_NumberOfPaymentsDelinquent4To11PaymentsPercentage
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

Name:	mtg_NumberOfPaymentsDelinquentAbstract
Namespace Prefix:	mtg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of loans in the primary default inventory.

+ References

No definition available.

+ Details

Name:	mtg_PrimaryDefaultInventory
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of loans in the primary default inventory that are in the claims received inventory.

+ References

No definition available.

+ Details

Name:	mtg_PrimaryDefaultInventoryClaimsReceivedInventory
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of loans in the primary default inventory that are in the claims received inventory.

+ References

No definition available.

+ Details

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

- Definition

The number of loans that have been in the primary default inventory for at least four months but no greater than eleven months as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	mtg_PrimaryDefaultInventoryFourToElevenMonths
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of loans that have been in the primary default inventory for at least four months but no greater than eleven months as of the balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

The number of loans in the primary default inventory that are in FEMA Individual Assistance Disaster Areas.

+ References

No definition available.

+ Details

Name:	mtg_PrimaryDefaultInventoryInFEMAIndividualAssistanceDisasterAreas
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of new notices reported in locations that FEMA has declared as Individual Assistance Disaster Areas (IADAs).

+ References

No definition available.

+ Details

Name:	mtg_PrimaryDefaultInventoryNewNoticesInFEMAIndividualAssistanceDisasterAreas
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of loans in the primary default inventory.

+ References

No definition available.

+ Details

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

- Definition

The number of loans that have been in the primary default inventory for three months or less as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	mtg_PrimaryDefaultInventoryThreeMonthsOrLess
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of loans that have been in the primary default inventory for three months or less as of the balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

The number of loans that have been in the primary default inventory for twelve months or more as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	mtg_PrimaryDefaultInventoryTwelveMonthsOrMore
Namespace Prefix:	mtg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of loans that have been in the primary default inventory for twelve months or more as of the balance sheet date.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

Name:	us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration