Loss Reserves - Default Inventory Reconciliation (Details) - loan	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Primary Default Inventory [Roll Forward]
Beginning delinquent inventory	46,556	50,282	62,633
New Notices	54,448	68,268	67,434
Cures	(60,511)	(61,094)	(65,516)
Paid claims	(5,750)	(9,206)	(12,367)
Rescissions and denials	(267)	(357)	(629)
Other items removed from inventory	(1,578)	(1,337)	(1,273)
Ending delinquent inventory	32,898	46,556	50,282

The number of loans in the primary default inventory.

No definition available.

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

Deductions from the primary default inventory resulting from loans in default being brought current.

No definition available.

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

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

No definition available.

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

Other deductions from the primary default inventory, including items removed for settlements

No definition available.

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

Deductions from the primary default inventory resulting from claims being paid, including those charged to a deductible or captive.

No definition available.

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

Deductions from the primary default inventory resulting from rescissions and denials.

No definition available.

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

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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