No definition available.

Name:	mtg_AgingOfPrimaryDefaultInventoryAbstract
Namespace Prefix:	mtg_
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 included in the pool insurance notice inventory as of the end of the reporting period.

+ References

No definition available.

+ Details

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

- 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

Additions to the primary default inventory resulting from new notices of defaulted loans.

+ References

No definition available.

+ Details

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

- 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

Primary Default Inventory, Other New Notices

+ References

No definition available.

+ Details

Name:	mtg_PrimaryDefaultInventoryOtherNewNotices
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

Primary Default Inventory Twelve Months Or More, Percent Of Primary Default Inventory In Default For Twelve Months Or More In Default For More Than Thirty Six Consecutive Months

+ References

No definition available.

+ Details

Name:	mtg_PrimaryDefaultInventoryTwelveMonthsOrMorePercentOfPrimaryDefaultInventoryInDefaultForTwelveMonthsOrMoreInDefaultForMoreThanThirtySixConsecutiveMonths
Namespace Prefix:	mtg_
Data Type:	num:percentItemType
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

- 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

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

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loss Reserves - Delinquent Items (Details) - loan	1 Months Ended		3 Months Ended		9 Months Ended
Loss Reserves - Delinquent Items (Details) - loan	Oct. 31, 2017	Oct. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Default notices for loans in IADAs		637
Other default notices		4,882
Total default notices		5,519	15,950	17,607	45,352	50,418
Aging of the Primary Default Inventory [Abstract]
3 months or less (in loans)			11,331	12,333	11,331	12,333		12,194
3 months or less (as a percent)			27.00%	24.00%	27.00%	24.00%		24.00%
4 - 11 months (in loans)			11,092	12,648	11,092	12,648		13,450
4 - 11 months (as a percent)			27.00%	25.00%	27.00%	25.00%		27.00%
12 months or more (in loans)			18,812	26,452	18,812	26,452		24,638
12 months or more (as a percent)			46.00%	51.00%	46.00%	51.00%		49.00%
Total primary default inventory (in loans)			41,235	51,433	41,235	51,433	41,317	50,282	52,558	62,633
Total primary default inventory (as a percent)			100.00%	100.00%	100.00%	100.00%		100.00%
Primary claims received inventory included in ending default inventory (in loans)			1,063	1,636	1,063	1,636		1,385
Primary claims received inventory included in ending default inventory (as a percent)			3.00%	3.00%	3.00%	3.00%		3.00%
Percent of default inventory in default for more than 36 months (as a percent)			45.00%	48.00%	45.00%	48.00%		47.00%
Number of payments delinquent [Abstract]
3 payments or less (in loans)			16,916	18,374	16,916	18,374		18,419
3 payments or less (as a percent)			41.00%	36.00%	41.00%	36.00%		36.00%
4 - 11 payments (in loans)			10,583	12,282	10,583	12,282		12,892
4 - 11 payments (as a percent)			26.00%	24.00%	26.00%	24.00%		26.00%
12 payments or more (in loans)			13,736	20,777	13,736	20,777		18,971
12 payments or more (as a percent)			33.00%	40.00%	33.00%	40.00%		38.00%
Total primary default inventory (in loans)			41,235	51,433	41,235	51,433	41,317	50,282	52,558	62,633
Total primary default inventory (as a percent)			100.00%	100.00%	100.00%	100.00%		100.00%
Pool insurance notice inventory (in loans)			1,426	1,979	1,426	1,979		1,883
Subsequent event
Liability for Claims and Claims Adjustment Expense [Line Items]
Default notices for loans in IADAs	3,394
Other default notices	4,549
Total default notices	7,943